serial: Coding style cleanup of struct serial_device

Do a simple cleanup of the struct serial_device and align it with
current coding style. Checkpatch now reports no errors.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>