fm10k: move error check

The error check and set_bit are placed in such a way that sparse (C=2)
warns:
.../fm10k_pci.c:1395:9: warning: context imbalance in 'fm10k_msix_mbx_pf' - different lock contexts for basic block

Which seems a little odd, but the code can obviously be moved
to where the variable is being set without changing functionality
at all, and it even seems to make a bit more sense with the check
closer to the set.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>