If the error matches, then a `+CIMI` command is sent to the device and
processing continues. However, the current error (for the `+CPIN?` command)
is not freed or propagated, fix that.
Fixes: 5c1fcbfe4a ("mtk-legacy: read IMSI if CPIN response is an error")
Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
