fm10k: Cleanup exception handling for mailbox interrupt

This patch addresses two issues.

First is the fact that the fm10k_mbx_free_irq was assuming msix_entries was
valid and that will not always be the case.  As such we need to add a check
for if it is NULL.

Second is the fact that we weren't freeing the IRQ if the mailbox API
returned an error on trying to connect.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Reviewed-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>