mfd: sm501: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case devm_kzalloc()
fails and returns NULL.

Fix this by adding a NULL check on *lookup*

This bug was detected with the help of Coccinelle.

Fixes: b2e63555 ("i2c: gpio: Convert to use descriptors")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>