serial: Prefer unsigned int to bare use of unsigned

Fix checkpatch warnings:
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'

Signed-off-by: Jinchao Wang <wjc@cdjrlc.com>
Link: https://lore.kernel.org/r/20210624021207.58059-1-wjc@cdjrlc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>