tty: n_gsm: Don't block input queue by waiting MSC

Currently gsm_queue() processes incoming frames and when opening
a DLC channel it calls gsm_dlci_open() which calls gsm_modem_update().
If basic mode is used it calls gsm_modem_upd_via_msc() and it
cannot block the input queue by waiting the response to come
into the same input queue.

Instead allow sending Modem Status Command without waiting for remote
end to respond. Define a new function gsm_modem_send_initial_msc()
for this purpose. As MSC is only valid for basic encoding, it does
not do anything for advanced or when convergence layer type 2 is used.

Fixes: 48473802 ("tty: n_gsm: fix missing update of modem controls after DLCI open")
Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
Link: https://lore.kernel.org/r/20250827123221.1148666-1-seppo.takalo@nordicsemi.no


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>