[media] cx231xx: get rid of warning: no previous prototype

drivers/media/usb/cx231xx/cx231xx-avcore.c:1071:5: warning: no previous prototype for 'stopAudioFirmware' [-Wmissing-prototypes]
drivers/media/usb/cx231xx/cx231xx-avcore.c:1076:5: warning: no previous prototype for 'restartAudioFirmware' [-Wmissing-prototypes]
drivers/media/usb/cx231xx/cx231xx-cards.c:689:6: warning: no previous prototype for 'cx231xx_reset_out' [-Wmissing-prototypes]
drivers/media/usb/cx231xx/cx231xx-cards.c:697:6: warning: no previous prototype for 'cx231xx_enable_OSC' [-Wmissing-prototypes]
drivers/media/usb/cx231xx/cx231xx-cards.c:701:6: warning: no previous prototype for 'cx231xx_sleep_s5h1432' [-Wmissing-prototypes]
drivers/media/usb/cx231xx/cx231xx-i2c.c:75:5: warning: no previous prototype for 'cx231xx_i2c_send_bytes' [-Wmissing-prototypes]

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