media: dvb/frontend.h: fix kernel-doc warnings

scripts/kernel-doc spouts multiple warnings, so fix them:

include/uapi/linux/dvb/frontend.h:399: warning: Enum value 'QAM_1024' not described in enum 'fe_modulation'
include/uapi/linux/dvb/frontend.h:399: warning: Enum value 'QAM_4096' not described in enum 'fe_modulation'
frontend.h:286: warning: contents before sections
frontend.h:780: warning: missing initial short description on line:
 * enum atscmh_rs_code_mode

Fixes: 8220ead8 ("media: dvb/frontend.h: document the uAPI file")
Fixes: 6508a50f ("media: dvb: add DVB-C2 and DVB-S2X parameter values")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Robert Schlabbach <robert_s@gmx.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>