- Fix assert/panic on receive when Jumbo Frames are enabled.

From the commit I made to ixl:
"It turns out that *_isc_rxd_available is supposed to return how many
packets are available to be cleaned on the rx ring. This patch removes
a section of code where if the budget argument is 1, the function would return
one if there was a descriptor available, not necessarily a packet.

This is okay in regular mtu 1500 traffic since the max frame size is less
than the configured receive buffer size (2048), but this doesn't work when
received packets can span more than one  descriptor, as is the case when the
mtu is 9000 and the receive buffer size is 4096."

- Fix possible Tx hang because *_isc_txd_credits_update returns incorrect result

From the commit by Krzysztof Galazka to ixl: "Function isc_txd_update_credits
called with clear set to false should return 1 if there are TX descriptors
already handled by HW. It was always returning 0 causing troubles with UDP TX
traffic."

PR:             231659
Reported by:    lev@
Approved by:	re (gjb@)
Sponsored by:   Intel Corporation