Bluetooth: Make UART draining logic consistent across drivers

There's no need to drain twice if NRF51_PM is enabled, the draining
shouldn't use rx_ready(), and it should happen after disabling
interrupts. Also rename uart.h to util.h to avoid conflicts with
include/uart.h and remove the left-over ISR prototype declaration.

Change-Id: Id38110dd38cf48edfe4a7b8e2e68cd358a9aa5ea
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>