Since dvb_frontend_detach() is not called in ttusb_dec_exit_dvb(),
which is called when the device is disconnected, dvb_frontend_free()
is not finally called.

This causes a memory leak just by repeatedly plugging and
unplugging the device.

Fix this issue by adding dvb_frontend_detach() to ttusb_dec_exit_dvb().

Link: https://lore.kernel.org/linux-media/20221117045925.14297-5-imv4bel@gmail.comSigned-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>