[media] si470x: fix compile warning

Tobias Lorenz <tobias.lorenz@gmx.net> has looked at this and agreed that the
'buf' variable could be removed.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>