V4L/DVB (8247): Fix a const pointer assignment error in the drx397xD demodulator driver

Fix an assignment of a const pointer to a non-const pointer in the drx397xD
demodulator driver.

This was introduced in patch eb9bd0e567365d4f607d32d8c41e201da65aa971.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>