net: stmmac: dwmac-qcom-ethqos: Use loopback_en for all speeds

It seems that this variable should be used for all speeds, not just
1000/100.

While at it refactor it slightly to be more readable, including fixing
the typo in the variable name.

Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>