V4L/DVB (9912): em28xx: fix/improve em28xx locking schema

Changes/fixes on em28xx dev->lock:

- em28xx_init_dev() were unlocking without a previous lock;

- some read ioctls need to lock after the removal of KBL, since a write
  may be happening at the same time an ioctl is reading;

- keep the device locked during all device initialization;

- lock/unlock while reading/writing registers.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>