enc28j60: Correct description of debug module parameter

The netif_msg_init() API takes on input the amount of bits to be set. The
description of debug parameter in the enc28j60 module is misleading in this
sense and passing 0xffff does not give an expected behaviour.

Fix the description of debug module parameter to show what exactly is expected.

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