[media] radio: set vfl_dir correctly to fix modulator regression

The vfl_dir field should be set to indicate whether a device can receive
data, output data or can do both. This is used to let the v4l core know
which ioctls should be accepted and which can be refused.
Unfortunately, when this field was added the radio modulator drivers were
not updated: radio modulators transmit and so vfl_dir should be set to
VFL_DIR_TX (or VFL_DIR_M2M in the special case of wl128x).
Because of this omission it is not possible to call g/s_modulator for these
drivers, which effectively renders them useless.
This patch sets the correct vfl_dir value for these drivers, correcting
this bug.
Thanks to Paul Grinberg for bringing this to my attention.

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