Bluetooth: fix wrong use of PTR_ERR() in btusb

PTR_ERR() returns a signed long type value which is limited by IS_ERR(),
it must be a negative number whose range is [-MAX_ERRNO, 0).

The bug here returns negative numbers as error codes, then check it by
"if (ret < 0)", but -PTR_ERR() is actually positive. The wrong use here
leads to failure as below, even panic.

[   12.958920] Bluetooth: hci0 command 0xfc8e tx timeout
[   14.961765] Bluetooth: hci0 command 0xfc8e tx timeout
[   16.964688] Bluetooth: hci0 command 0xfc8e tx timeout
[   20.954501] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   22.957358] Bluetooth: hci0 command 0xfc8e tx timeout
[   30.948922] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   32.951780] Bluetooth: hci0 command 0xfc8e tx timeout
[   40.943359] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   42.946219] Bluetooth: hci0 command 0xfc8e tx timeout
[   50.937812] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   52.940670] Bluetooth: hci0 command 0xfc8e tx timeout
[   60.932236] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   62.935092] Bluetooth: hci0 command 0xfc8e tx timeout
[   70.926688] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   72.929545] Bluetooth: hci0 command 0xfc8e tx timeout
[   80.921111] Bluetooth: hci0 sending Intel patch command (0xfc8e) failed (-110)
[   82.923969] Bluetooth: hci0 command 0xfc2f tx timeout
[   90.915542] Bluetooth: hci0 sending Intel patch command (0xfc2f) failed (-110)
[   92.918406] Bluetooth: hci0 command 0xfc11 tx timeout
[  100.909955] Bluetooth: hci0 sending Intel patch command (0xfc11) failed (-110)
[  102.912858] Bluetooth: hci0 command 0xfc60 tx timeout
[  110.904394] Bluetooth: hci0 sending Intel patch command (0xfc60) failed (-110)
[  112.907293] Bluetooth: hci0 command 0xfc11 tx timeout
[  120.898831] Bluetooth: hci0 exiting Intel manufacturer mode failed (-110)
[  120.904757] bluetoothd[1030]: segfault at 4 ip 00007f8b2eb55236 sp 00007fff53ff6920 error 4 in bluetoothd[7f8b2eaff000+cb000]

Signed-off-by: Adam Lee <adam.lee@canonical.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>