[ARM] 3479/1: Corgi SSP: Fix potential concurrent access problem

Patch from Richard Purdie

corgi_ssp_probe() should not access GPDR directly but should use
pxa_gpio_mode() which has appropriate locking and other safeguards.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>