net/macb: improve big endian CPU support

The commit a50dad35 (net: macb: Add big endian CPU support) converted I/O
accessors to readl_relaxed() and writel_relaxed() and consequentially broke
MACB driver on AVR32 platforms such as ATNGW100.

This patch improves I/O access by checking endiannes first and use the
corresponding methods.

Fixes: a50dad35 (net: macb: Add big endian CPU support)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>