net: macb: Add NULL check for PCLK and HCLK

net: macb: Add NULL check for PCLK and HCLK

Both PCLK and HCLK are "required" clocks according to macb devicetree
documentation. There is a chance that devm_clk_get doesn't return a
negative error but just a NULL clock structure instead. In such a case
the driver proceeds as usual and uses pclk value 0 to calculate MDC
divisor which is incorrect. Hence fix the same in clock initialization.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>