backlight: pwm_bl: 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).

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Lee Jones <lee.jones@linaro.org>