mfd: asic3: Fix potential null pointer dereference

We previously assumed 'mem_sdio' could be null but it is
dereferenced in ioremap(). Add a check to avoid a potential
null pointer dereference error.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>