cxgb4: handle serial flash interrupt

If SF bit is not cleared in PL_INT_CAUSE, subsequent non-data
interrupts are not raised.  Enable SF bit in Global Interrupt
Mask and handle it as non-fatal and hence eventually clear it.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>