net: macb: fix build warning

We are getting build warning about:
macb.c:2889:13: warning: 'tx_clk' may be used uninitialized in this function
macb.c:2888:11: warning: 'hclk' may be used uninitialized in this function

In reality they are not used uninitialized as clk_init() will initialize
them, this patch will just silence the warning.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>