fm10k/base: remove useless variable

fm10k/base: remove useless variable

The reference to err_no was left around after an old re-factor. We never
use this value again, and the macros called on the function appear to
have no relevant side effect I could see. Discovered via cppcheck
fm10k_mbx.c:1312: (style) Variable 'err_no' is assigned a value that is never used.

This occurred because a previous commit refactored and removed all used
references to err_no.

Signed-off-by: Wang Xiao W <xiao.w.wang@intel.com>