amd-xgbe: Fix debugfs compatibility change with kstrtouint

The initial change from sscanf to kstrtouint broke backward
compatbility by using a base of "0" in the kstrtouint call.
This allowed for entering decimal, hexadecimal or octal as
input where previously the sscanf always interpreted the input
as hexadecimal.  Additionally, -EIO was returned on error prior
to this change and now it is whatever the error value that is
returned by kstrtouint.

Change the base value of the kstrtouint from 0 to 16 and return
-EIO on error.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>