amd-xgbe: Fix possible uninitialized variable

The debugfs support in the driver uses a common routine to write the
debugfs values. In this routine, if the input file position is non-zero
then the write routine will not return an error and an output parameter
will not have been set. Because an error isn't returned an uninitialized
value will be written into a register.

Fix the common write routine to return an error if the input file position
is non-zero, which will propagate back to the caller.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>