net/macb: check if macb_config present

The commit 98b5a0f4 introduces jumbo frame support, but also it assumes
that macb_config present which is not always true.

The configuration without macb_config fails to boot.

 Unable to handle kernel NULL pointer dereference at virtual address 00000010
 ptbr = 90350000 pgd = 00000000
 Oops: Kernel access of bad area, sig: 11 [#1]
 FRAME_POINTER chip: 0x01f:0x1e82 rev 2
 Modules linked in:
 CPU: 0 PID: 1 Comm: swapper Not tainted 4.2.0-rc3-next-20150723+ #13
 task: 91c26000 ti: 91c28000 task.ti: 91c28000
 PC is at macb_probe+0x140/0x61c

Fixes: 98b5a0f4 (net: macb: Add support for jumbo frames)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>