[media] mxl111sf: fix sparse warnings

drivers/media/usb/dvb-usb-v2/mxl111sf.c:34:5: warning: symbol 'dvb_usb_mxl111sf_isoc' was not declared. Should it be static?
drivers/media/usb/dvb-usb-v2/mxl111sf.c:38:5: warning: symbol 'dvb_usb_mxl111sf_spi' was not declared. Should it be static?
drivers/media/usb/dvb-usb-v2/mxl111sf.c:46:5: warning: symbol 'dvb_usb_mxl111sf_rfswitch' was not declared. Should it be static?
drivers/media/usb/dvb-usb-v2/mxl111sf.c:890:22: warning: symbol 'mxl111sf_i2c_algo' was not declared. Should it be static?

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>