net/macb: unify clock management

Most of the functions from the Common Clk Framework handle NULL pointer as
input argument.

Since the TX clock is optional, we now set tx_clk to NULL value
instead of ERR_PTR(-ENOENT) when this clock is not available. This simplifies
the clock management and avoid the need to test tx_clk value.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>