sky2: fix missing register reset on error path in sky2_test_msi()

In sky2_test_msi() the temporarily set SW IRQ in B0 register is not reset in case
that request_irq() fails.
With this patch we only set the interrupt mask if request_irq() was successful.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>