media: m5mols: Check function pointer in m5mols_sensor_power

clang static analysis reports this error

m5mols_core.c:767:4: warning: Called function pointer
  is null (null dereference) [core.CallAndMessage]
    info->set_power(&client->dev, 0);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In other places, the set_power ptr is checked.
So add a check.

Fixes: bc125106 ("[media] Add support for M-5MOLS 8 Mega Pixel camera ISP")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>