ionic: refactor use of ionic_rx_fill()

The same pre-work code is used before each call to
ionic_rx_fill(), so bring it in and make it a part of
the routine.

Signed-off-by: Neel Patel <neel@pensando.io>
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>