V4L/DVB (11917): Fix firmware load for DVB-T @ 6MHz bandwidth for xc3028/xc3028L

The only two countries that are known to use 6MHz bandwidth are Taiwan
and Uruguay. Both use QAM subcarriers at OFTM.

This patch fixes the firmware load for such countries, where the
required firmware is the QAM one.

This also confirms the previous tests where it was noticed that the 6MHz
QAM firmware doesn't work for cable. So, this patch also removes support
for DVB-C, instead of just printing a warning.

Thanks to Terry Wu <terrywu2009@gmail.com> for pointing this issue and
to Andy Walls <awalls@radix.net> for an initial patch for this fix.

Cc: Terry Wu <terrywu2009@gmail.com>
Acked-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>