cxgb4 : Disable recursive mailbox commands when enabling vi

Enabling a Virtual Interface can result in an interrupt during the processing
 of the VI Enable command and, in some paths, result in an attempt to issue
another command in the interrupt context, eventually crashing the system. Thus,
 we disable interrupts during the course of the VI Enable command and ensure
enable doesn't sleep.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>