enic: Clean up: Add wrapper routines for firmware devcmd calls

Add wrapper routines that issue devcmds to firmware and ensure that a
devcmd lock is held for each devcmd call.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>