When CONFIG_SND is not enabled, radio-sf16fmr2 build fails with:

so make this driver depend on SND.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: linux-media@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>