staging: media: atomisp: fix build errors when PM is disabled

The function atomisp_restore_iumit_reg is unused when PM is disabled.
Adding __maybe_unused to the function definition avoids a warning.

The function atomisp_mrfld_power_down is defined only when PM is
enabled. So in atomisp_pci_probe, it should be called only when PM is
enabled.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>