tty: serial: 8250: Add parentheses to macro

This patch fixes a checkpatch warning caused by missing parentheses
in the definition of a macro.
Furthermore redundant parentheses are removed in an assignment.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: linux-kernel@i4.cs.fau.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>