fm10k: reduce scope of the err variable

Reduce the scope of the err local variable in the fm10k_dcbnl_ieee_setets
function.

This was detected using cppcheck, and resolves the following style
warning:

[fm10k_dcbnl.c:37]: (style) The scope of the variable 'err' can be reduced.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>