stmmac: remove custom implementation of print_hex_dump()

There is a kernel helper to dump buffers in a hexdecimal format. This patch
substitutes the open coded function by calling that helper.

The output is slightly changed:
 - no lead space
 - ASCII part will be printed along with the dump
 - offset is longer than 3 characters (now 8)

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>