[media] em28xx: fix deadlock when unplugging and replugging a DVB adapter

This fixes the deadlock that occurs with either multiple PCTV 290e adapters or when a single PCTV 290e adapter is replugged.

For DVB devices, the device lock must now *not* be held when adding/removing either a device or an extension to the respective lists. (Because em28xx_init_dvb() will want to take the lock instead).

Conversely, for Audio-Only devices, the device lock *must* be held when adding/removing either a device or an extension to the respective lists.

Signed-off-by: Chris Rankin <rankincj@yahoo.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>