enc28j60: Improves interface with native IP stack

Tranmission:  Consider that fragments are no adjacent.

Reception:    Consider receiving data in more than one fragment.

SPI routines: Align SPI fragment size with default buffer fragment
              size (128 bytes) for better performance.

Change-Id: I51fc25d8540c36f3719e617a6f33cdea3f63032c
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@intel.com>