media: dvb-frontends: add missing MODULE_DESCRIPTION() macros

With ARCH=x86, make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/media/dvb-frontends/au8522_decoder.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/media/dvb-frontends/mb86a16.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>