[ARM] pxa: remove now unnecessary pxa_gpio_mode() calls in ac97

Now most (if not all) PXA platforms have been switched to the new MFP
API, it's rather safe to remove these unnecessary pxa_gpio_mode() calls
in pxa2xx-ac97-lib.c now.

Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>