media: mxl111sf: remove redundant assignment to index

Variable index is set to zero and then set to zero again
a few lines later in a for loop initialization. Remove the
redundant setting of index to zero. Cleans up the clang
warning:

drivers/media/usb/dvb-usb-v2/mxl111sf-i2c.c:519:3: warning: Value
stored to 'index' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>