gianfar: Factor out enabling/disabling of hw interrupts

Throughout the code there are places where the controller's
hw interrupt sources need to get disabled/enabled (masked/
un-masked) all at once.  The recommendation for disabling
the interrupts is to clear the ievent first then the imask
register (not the other way around).
Use the gfar_ints_enable/disable() helpers to make these
operations consistent.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>