V4L/DVB (11799): xc5000: don't load firmware until a tuning request is made

Defer loading of the xc5000 firmware until it is actually needed.  This helps
on distros that have hald, which results in the device not being available
for use for around ten seconds in cases where the i2c bus is slow (such as
the HVR-950Q).  Also, the firmware load isn't really useful since we
immediately put the device to sleep afterward, which means a firmware reload
will be required anyway.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>