fm10k: reduce the scope of the err variable

Reduce the scope of the local variable err in the fm10k_detach_subtask
function.

This was detected by cppcheck and resolves the following warning
produced by that tool:

[fm10k_pci.c:403]: (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>