uqmi: skip registration state check if not supported

uqmi: skip registration state check if not supported

The Quectel RG255C modem used in the Teltonika RUT976 does not support
the 'Get Serving System' QMI command, returning "Invalid QMI command".
Without this change, the script would fail even though the connection
could be established successfully.

This patch skips the registration state check if the command is not
supported and relies on subsequent checks instead.

Signed-off-by: Til Kaiser <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20933
Signed-off-by: Hauke Mehrtens <[email protected]>