gianfar: Fix boot hangs while bringing up gianfar ethernet

Ira Snyder found that commit 8c7396ae


"gianfar: Merge Tx and Rx interrupt for scheduling clean up ring" can
cause hangs. It's because there was removed clearing of interrupts in
gfar_schedule_cleanup() (which is called by an interrupt handler) in
case when netif scheduling has been disabled. This patch brings back
this action and a comment.

Reported-by: Ira Snyder <iws@ovro.caltech.edu>
Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Bisected-by: Ira Snyder <iws@ovro.caltech.edu>
Tested-by: Peter Korsgaard <jacmet@sunsite.dk>
Tested-by: Ira Snyder <iws@ovro.caltech.edu>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>