net: eth: altera: Change reset_mac failure message masks from err to dbg

This debug output is not really an error message since mac reset can fail
if the phy clocks are gated, specifically when the phy has been placed in
a powered down or isolation mode. The netdev output masks were changed from
err to dbg, and comments added in the code.

Signed-off-by: Vince Bridgers <vbridger@opensource.altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>