media: mmp-driver: make two functions static

Those functions are used only internally:

  CC      drivers/media/platform/marvell-ccic/mmp-driver.o
drivers/media/platform/marvell-ccic/mmp-driver.c:186:6: warning: no previous prototype for ‘mcam_ctlr_reset’ [-Wmissing-prototypes]
 void mcam_ctlr_reset(struct mcam_camera *mcam)
      ^~~~~~~~~~~~~~~
drivers/media/platform/marvell-ccic/mmp-driver.c:217:6: warning: no previous prototype for ‘mmpcam_calc_dphy’ [-Wmissing-prototypes]
 void mmpcam_calc_dphy(struct mcam_camera *mcam)
      ^~~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>