can: slcan: allow reception of short error messages

Allows slcan to receive short messages (typically errors) from the serial
interface.

When error support was added to slcan protocol in
b32ff466 ("can: slcan: extend the protocol
with error info") the minimum valid message size changed from 5 (minimum
standard can frame tIII0) to 3 ("e1a" is a valid protocol message, it is
one of the examples given in the comments for slcan_bump_err() ), but the
check for minimum message length prodicating all decoding was not adjusted.
This makes short error messages discarded and error frames not being
generated.

This patch changes the minimum length to the new minimum (3 characters,
excluding terminator, is now a valid message).

Signed-off-by: Carlos Sanchez <carlossanchez@geotab.com>
Fixes: b32ff466 ("can: slcan: extend the protocol with error info")
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://patch.msgid.link/20250520102305.1097494-1-carlossanchez@geotab.com


Cc: stable@vger.kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>