Bluetooth: btusb: avoid unused function warning

The btusb_rtl_cmd_timeout() function is used inside of an
ifdef, leading to a warning when this part is hidden
from the compiler:

drivers/bluetooth/btusb.c:530:13: error: unused function 'btusb_rtl_cmd_timeout' [-Werror,-Wunused-function]

Use an IS_ENABLED() check instead so the compiler can see
the code and then discard it silently.

Fixes: d7ef0d1e ("Bluetooth: btusb: Use cmd_timeout to reset Realtek device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>