staging: dgap: removes redundant null check and change paramter for dgap_maxcps_room()

Null checks for tty, un and ch are already done by caller,
so replace parameter "tty" with "ch" and "un".

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>