Blackfin arch: remove useless SSYNC() in irq priority code

 - remove SSYNC() left over from irq init split
 - do not force SSYNC() when masking/unmasking IRQs in the SIC
   as any order enforced by the hardware should already be enforced
   by software

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>