sfc: make ef100 xmit_more handling look more like ef10's

This should cause no functional change; merely make there only be one
 design of xmit_more handling to understand.  As with the EF10/Siena
 version, we set tx_queue->xmit_pending when we queue up a TX, and
 clear it when we ring the doorbell (in ef100_notify_tx_desc).
While we're at it, make ef100_notify_tx_desc static since nothing
 outside of ef100_tx.c uses it.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>