mmc: wmt-sdmmc: Fix NULL pointer dereference

'of_id' is dereferenced before NULL pointer check. Move it to
after the check.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Chris Ball <chris@printf.net>