V4L/DVB (7439): tuner-xc2028: Adds an option to allow forcing to load an specific firmware name

There are a number of different firmware versions and variants, shipped
together with boards.

This patch adds an extra parameter to the tuner-xc2028 to allow specifying for
an specific firmware name to be loaded. This helps to test for a firmware that
better fits some board.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>