fm10k: remove err_no reference in fm10k_mbx.c

The reference to err_no was left around after a previous code refactor.
We never use the value, and it doesn't seem to be used in side a hidden
macro reference. Discovered via cppcheck.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>