mmc: sdhci-spear: Fix NULL pointer dereference

pdata could be NULL if cd_gpio = -1. Dereference pdata only
if it is not NULL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>