s390/qeth: cache link_info for ethtool

Since
commit e6e771b3 ("s390/qeth: detach netdevice while card is offline")
there was a timing window during recovery, that qeth_query_card_info could
be sent to the card, even before it was ready for it, leading to a failing
card recovery. There is evidence that this window was hit, as not all
callers of get_link_ksettings() check for netif_device_present.

Use cached values in qeth_get_link_ksettings(), instead of calling
qeth_query_card_info() and falling back to default values in case it
fails. Link info is already updated when the card goes online, e.g. after
STARTLAN (physical link up). Set the link info to default values, when the
card goes offline or at STOPLAN (physical link down). A follow-on patch
will improve values reported for link down.

Fixes: e6e771b3

 ("s390/qeth: detach netdevice while card is offline")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Thorsten Winkler <twinkler@linux.ibm.com>
Link: https://lore.kernel.org/r/20220805155714.59609-1-wintera@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>