drivers: serial: Make serial_initialize return int

serial_initialize is called only during the common init sequence, after
relocation (in common/board_r.c). Because it has a void return value, it
has to wrapped in initr_serial. In order to be able to get rid of this
indirection, make serial_initialize return int.

Remove extern from prototype in order to silence the following checkpatch
warning:
check: extern prototypes should be avoided in .h files

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>