Bluetooth: btusb: Use wait_on_bit_timeout() for BTUSB_BOOTING

The wait_on_bit_timeout() is a simpler and race-free way of waiting for
a bit to be cleared than the current code in btusb.c. This patch updates
the code to use the helper function (its btusb copy - to be later
updated to use a global one).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>