sfc: Remove TX completions from adaptive IRQ scoring

RX and TX completions on the same event queue are generally not associated
with the same flows.  The inclusion of TX completions in the adaptive IRQ
score is more of a source of noise rather than useful feedback.  Therefore,
do not include them in the score, and adjust the default threshold scores
down.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>