Revert "n_gsm: race between ld close and gsmtty open"

This reverts commit c284ee2c.  Turns out
the locking was incorrect.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Chao Bi <chao.bi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>