enc28j60: hardware errata #6 PKTIF is unreliable

The Receive Packet Pending Interrupt Flag (EIR.PKTIF) does not
reliably/accurately report the status of pending packets.
Reception fails intermittently. The EPKTCNT register is used instead.

Change-Id: I599c3b4df1b74d769dd7104e5c6e253d9cf63bd4
Signed-off-by: Juan Manuel Cruz Alcaraz <juan.m.cruz.alcaraz@intel.com>