V4L/DVB (6070): Fix a warning at dvb_net

static function dvb_net_sec declares input arg "pkt" as u8. However, the
same argument at dvb_net_sec_callback is defined as "const u8". When
calling dvb_net_sec, this is casted as just "u8".

gcc 4.2.1 generates a warning about that:

  CC [M]  drivers/media/dvb/dvb-core/dvb_net.o
drivers/media/dvb/dvb-core/dvb_net.c: In function "dvb_net_sec_callback":
drivers/media/dvb/dvb-core/dvb_net.c:905: warning: passing argument 2 of
		"dvb_net_sec" discards qualifiers from pointer target type

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