V4L/DVB (8098): xc5000: add module option to load firmware during driver attach

The xc5000 firmware download process can be slow on certain devices.

Add a module option, "init_fw" to perform the firmware download during
xc5000_attach(), which would get the firmware download over with well
before any tune request is made.

When this option is enabled, it will reduce the time it takes to get a
signal lock during the first tune after driver load, but will add a few
seconds to the driver initialization.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>