serial/uart.h: no need to check driver_api being NULL

The device_get_binding() no longer returns pointers
to device struct when driver_api is NULL. Therefore,
there is no need to check for driver_api being NULL
in the serial driver wrapper functions.

Change-Id: I018a409324d7c1ae83c699a3ebf30f2f2abfb3b3
Signed-off-by: Daniel Leung <daniel.leung@intel.com>