[SCSI] megaraid_sas: function pointer for disable interrupt

This patch adds function pointer to invoke disable interrupt for
xscale and ppc IOP based controllers. Removes old implementation that checks
for controller type in megasas_disable_intr.

Signed-off-by: Sumant Patro <Sumant.Patro@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>