amd-xgbe: Allow rx-frames coalescing to be changed anytime

Currently the device must be down in order to update the rx-frames
coalescing setting because the interrupt indicator is set in the
descriptor data during initialization. Allow this setting to be changed
while the device is up by moving the interrupt decision into the
descriptor reset function and base the decision off of the supplied
descriptor index value.

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