media: dvb-frontends/stv0910: implement diseqc_send_burst

This implements the diseqc_send_burst frontend op to support sending
mini-DISEQC bursts. Picked up from dddvb's driver package where this
specific block was disabled via #if 0/#endif, but is still working
according to feedback from upstream, so add it.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>