staging/easycap: Eliminate BKL

No locking is required for normal operation of the driver, but locking
is needed to prevent an Oops during some hot-unplugging scenarios.  The
BKL is replaced here by mutex locks together with traps to detect null
pointers following asynchronous device disconnection.

Signed-off-by: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>