[media] media: dvb-core move fe exit flag from fepriv to fe for driver access

Some fe drivers attempt to access the device for power control from
their release routines. When release routines are called after device
is disconnected, the attempts fail. fe drivers should avoid accessing
the device, from their release interfaces when called from disconnect
path. dvb-frontend maintains exit flag to keep track when fe device is
disconnected in its private data structures. Export the flag in fe to
enable drivers to check the device status from their release interfaces.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>