Add escape before asterisk to fix those warnings:

Documentation/linux_tv/media/dvb/FE_GET_EVENT:47: WARNING: Inline emphasis start-string without end-string.
Documentation/linux_tv/media/v4l/media-ioc-enum-links:78: WARNING: Inline emphasis start-string without end-string.
Documentation/linux_tv/media/v4l/media-ioc-enum-links:87: WARNING: Inline emphasis start-string without end-string.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>