mfd: pm8921: Potential NULL dereference in pm8921_remove()

We assume that "pmic" could be NULL and then dereference it two lines
later.  I fix this by moving the dereference inside the NULL check.

Fixes: c013f0a5 ('mfd: Add pm8xxx irq support')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>