V4L/DVB (9771): tuner-xc2028: fix a small warning

/home/mauro/v4l-dvb/v4l/tuner-xc2028.c: In function 'xc2028_sleep':
/home/mauro/v4l-dvb/v4l/tuner-xc2028.c:1111: warning: 'return' with no value, in function returning non-void

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