amd-xgbe: Fix Rx coalescing reporting

The Rx coalescing value is internally converted from usecs to a value
that the hardware can use. When reporting the Rx coalescing value, this
internal value is converted back to usecs. During the conversion from
and back to usecs some rounding occurs. So, for example, when setting an
Rx usec of 30, it will be reported as 29. Fix this reporting issue by
keeping the original usec value and using that during reporting.

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