Revert "n_gsm.c: add tx_lock in gsm_send"

This reverts commit f96f7f7f, at the
request of Jin.

Cc: xiaojin <jin.xiao@intel.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>