Revert "tty: n_gsm: avoid call of sleeping functions from atomic context"

Revert "tty: n_gsm: avoid call of sleeping functions from atomic context"

This reverts commit 9fa881837ae86836387e4d2197a347c8df5fd997.

The above commit is reverted as the usage of tx_mutex seems not to solve
the problem described in 9fa881837ae8 ("tty: n_gsm: avoid call of sleeping
functions from atomic context") and just moves the bug to another place.

Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20221008110221.13645-2-pchelkin@ispras.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>