serial: mxs-auart: don't use GPIOF_* with gpiod_get_direction

The documentation was wrong, gpiod_get_direction() returns 0/1 instead
of the GPIOF_* flags. The docs were fixed with commit 94fc7309
("gpio: correct docs about return value of gpiod_get_direction"). Now,
fix this user (until a better, system-wide solution is in place). This
also means we can drop the deprecated use of 'linux/gpio.h'. Yay!

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>