net/at91_ether: avoid NULL pointer dereference

The at91_ether driver calls macb_mii_init passing a 'struct macb'
structure whose tx_clk member is initialized to 0. However,
macb_handle_link_change() expects tx_clk to be the result of
a call to clk_get, and so IS_ERR(tx_clk) to be true if the clock
is invalid. This causes an oops when booting Linux 3.14 on the
csb637 board. The following changes avoids this.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>