rt2x00: Unify GPIO register field namings

The various rt2x00 drivers use different methods to name the different
GPIO register fields indicating the GPIO pin value and the fields
indicating the direction.
Start using a unified naming scheme for the GPIO register fields:
	- <csr>_VAL<x> for fields indicating the GPIO pin value.
	- <csr>_DIR<x> for fields indicating the GPIO pin direction.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo Van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>