drivers: ethernet: enc28j60: disable/enable interrupts to avoid races

Currently, there is a small race window where we can miss an interrupt.
Right after we're done reading the RX buffer but just before decrementing
the RX counter to zero, the ENC28J60 may receive a packet. The chip will
raise an interrupt, but the line is still asserted. That means that the
callback will not be invoked since it is edge-triggered.

To avoid that, disable interrupts on the chip itself before processing
the RX buffer.

In fact, the ENC28J60 datasheet specifically says:

	"After an interrupt occurs, the host controller should
	clear the global enable bit for the interrupt pin before
	servicing the interrupt. Clearing the enable bit will
	cause the interrupt pin to return to the non-asserted
	state (high). Doing so will prevent the host controller
	from missing a falling edge should another interrupt
	occur while the immediate interrupt is being serviced.
	After the interrupt has been serviced, the global enable
	bit may be restored. If an interrupt event occurred while
	the previous interrupt was being processed, the act of
	resetting the global enable bit will cause a new falling
	edge on the interrupt pin to occur."

This is also what is being done in the Linux driver [1].

[1] https://elixir.bootlin.com/linux/v6.11.2/source/drivers/net/ethernet/microchip/enc28j60.c#L1126



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