tty: n_gsm: increase malformed counter for malformed control frames

tty: n_gsm: increase malformed counter for malformed control frames

The malformed counter in gsm_mux is already increased in case of errors
detected in gsm_queue() and gsm1_receive(). gsm_dlci_command() also
detects a case for a malformed frame but does not increase the malformed
counter yet.

Fix this by also increasing the gsm_mux malformed counter in case of a
malformed frame in gsm_dlci_command().
Note that the malformed counter is not yet exposed and only set internally.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-5-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>