media: dvb: fix DVB_MMAP symbol name

CONFIG_DVB_MMAP was misspelled either as CONFIG_DVB_MMSP
or DVB_MMAP, so it had no effect at all. This fixes that,
to make it possible to build it again.

Fixes: 4021053e ("media: dvb-core: make DVB mmap API optional")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>