drivers: ethernet: eth_enc28j60: do not check PKTIF on interrupt

The enc28j60 errata sheet says:

	"The Receive Packet Pending Interrupt Flag
	(EIR.PKTIF) does not reliably/accurately report
	the status of pending packets."

	"In the Interrupt Service Routine, if it is unknown if
	a packet is pending and the source of the interrupt
	is unknown, switch to Bank 1 and check the value
	in EPKTCNT.
	If polling to see if a packet is pending, check the
	value in EPKTCNT."

A workaround has already been implemented inside of eth_enc28j60_rx().
But checking PKTIF before calling eth_enc28j60_rx() completely defeats
the purpose of the workaround. Do not check it.

Moreover, clearing ENC28J60_BIT_EIR_PKTIF is useless since it is
automatically cleared once all packets are read. So remove that check
and clarify comment.

Also please refer to the Linux driver [1].

[1] https://elixir.bootlin.com/linux/v6.11.2/source/drivers/net/ethernet/microchip/enc28j60.c#L1090



Signed-off-by: Xavier Ruppen <xruppen@gmail.com>