[media] cxd2820r: Fix an incorrect modulation type bitmask

Fix an incorrect modulation type bitmask. This allows QAM256 also to be
correctly reported.

Signed-off-by: Janne Huttunen <jahuttun@gmail.com>
Acked-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>