staging/easycap: Add option to show conspicuous indication of signal loss

A new module parameter turns on the option of displaying a testcard when
the analogue input signal is lost (more precisely: when the hardware
detects no field/frame synchronization).  This feature has been requested
in the context of security cameras used at night.

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