V4L/DVB (6112): cx25840: use a workqueue to load the firmware

Loading the firmware using the i2c bit-banging code blocks the kernel.
Move the firmware load code into a workqueue so that it plays well with
other processes.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>