Bluetooth: hci_ath: Replace mdelay with msleep in ath_wakeup_ar3k

ath_wakeup_ar3k() is never called from atomic context.

It is only called by ath_hci_uart_work() that is only called in
ath_open() via INIT_WORK().
All of the above functions do not enter atomic context along the way.

Despite never getting called from atomic context, ath_wakeup_ar3k() calls
mdelay() for busy wait.
That is not necessary and can be replaced with msleep to avoid busy wait.

This is found by a static analysis tool named DCNS written by myself.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>