mfd: mfd-core: Protect against NULL call-back function pointer

[ Upstream commit b195e101580db390f50b0d587b7f66f241d2bc88 ]

If a child device calls mfd_cell_{en,dis}able() without an appropriate
call-back being set, we are likely to encounter a panic.  Avoid this
by adding suitable checking.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>