Input: MT - document new 'flags' argument of input_mt_init_slots()

Fixes new kernel-doc warning in input-mt.c:

Warning(drivers/input/input-mt.c:38): No description found for parameter
'flags'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>