cxgb4: always sync access when flashing PHY firmware

Access to on-chip memory for flashing PHY firmware must always
be synchronized. So, ensure the callers take on-chip memory lock.

Also fixes following sparse warning:
sge.c:1641:26: warning: context imbalance in 't4_load_phy_fw' -
different lock contexts for basic block

Fixes: 01b69614 ("cxgb4: Add PHY firmware support for T420-BT cards")
Fixes: 4ee339e1

 ("cxgb4: add support to flash PHY image")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>