enc28j60: Switch to use module_spi_driver() macro

Eliminate some boilerplate code by using module_spi_driver() instead of
->init() / ->exit(), moving the salient bits from ->init() into ->probe().

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