serial: core: Fix Coding Style

This patch fixes the following checkpatch error and warning:
  1. space required after ','
  2. Missing a blank line after declarations

Signed-off-by: Tamseel Shams <m.shams@samsung.com>
Link: https://lore.kernel.org/r/20200716115438.9967-1-m.shams@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>