staging: greybus: uart.c: change 'unsigned' to 'unsigned int'

Fixes checkpatch.pl warning:
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>