V4L/DVB (11624): cx18: Toggle the AI1 mux when changing the CX18_AUDIO_ENABLE register

Toggle the AI1 mux when changing the CX18_AUDIO_ENABLE register.  It's hard to
reliably tell when we have written to this register successfully unless we
change some bits we know we can read back.  The AI mux bits always read back
what we wrote to them, so force them to toggle whenever we have to write to
the register, so we can tell we wrote to the register successfully.

This change was prompted by users experiencing broadcast audio decoding
problems after the cx18 module loads for the first time.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>