net/macb: fix probe sequence to setup clocks earlier

As accessing the peripheral registers need the clocks to be set, we have to
enable them as soon as possible. Their configuration depend on the type of
device used and determined by the DT compatible string. That lead to add
another initialization function in the DT configuration structure.
As the device private structure length depend on an information read in the
registers, we have to store the clock pointers in temporary variables before
feeding the structure fields.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>