net: dsa: bcm_sf2: Utilize mask clear/set helpers in bcm_sf2_intr_disable

And while at it, remove the unecessary writing of zeroes to the CPU_MASK_CLEAR
register since it has no functional use.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>