enc28j60: Amend comments by fixing typos, adding periods, etc

Amend comments in the code:
 - adding periods to the multi-line comments
 - fixing typos
 - capitalize first word in the sentences
 - etc

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