tty/n_gsm.c: get gsm->num after gsm_activate_mux

gsm->num is the index of gsm_mux[], it's invalid before calling gsm_activate_mux.

Signed-off-by: xinhui.pan <xinhuiX.pan@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>