staging: gdm724x: gdm_tty: fix tty api build warnings

This fixes the build warnings due to changes in the tty api.  Note, I
don't know if the reference counting is correct, but at least there are
no more build warnings.

Also the tty code needs to be reworked, no static arrays of tty devices
and the like...

Cc: Won Kang <wonkang@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>