[media] dib7000m/dib7000p: Add support for TRANSMISSION_MODE_4K

Fixes several warnings:

drivers/media/dvb/frontends/dib7000m.c: In function ‘dib7000m_set_channel’:
drivers/media/dvb/frontends/dib7000m.c:808:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000m.c:869:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000m.c: In function ‘dib7000m_tune’:
drivers/media/dvb/frontends/dib7000m.c:1023:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000m.c:1033:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000m.c:1043:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000p.c: In function ‘dib7000p_set_channel’:
drivers/media/dvb/frontends/dib7000p.c:720:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000p.c:773:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000p.c: In function ‘dib7000p_tune’:
drivers/media/dvb/frontends/dib7000p.c:997:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000p.c:1007:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’
drivers/media/dvb/frontends/dib7000p.c:1017:3: warning: case value ‘255’ not in enumerated type ‘fe_transmit_mode_t’

The drivers were prepared to support 4K carriers, but as the define were added
later, they were using a "magic" value of 255.

Cc: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>