igb: add function to handle mailbox lock

Both the read and write mailbox functions need to acquire the mailbox lock.
Since that is the case we might as well combine both of the procedures into
one function so it is easier to maintain.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>