sierra_net: Skip validating irrelevant fields for IDLE LSIs

When the context is deactivated, the link_type is set to 0xff, which
triggers a warning message, and results in a wrong link status, as
the LSI is ignored.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>