enc28j60: reduce the number of spi transfers in enc28j60_set_bank()

A major source of overhead in the enc28j60 driver is the SPI transfers. Each
SPI transfer entails two kernel thread context switches. One major source of
SPI transfers is the enc28j60_set_bank() functions which runs before every
register access. This patch reduces the number of SPI transfers that
enc28j60_set_bank() performs in two ways:

  1. removes unnecessary bank switch for the registers that are present in all
	 banks

  2. when switching from banks 0 or 3 to banks 1 or 2 (i.e. only one bit
	 changes) enc28j60_set_bank() does only one SPI transfer instead of two

According to my tests these changes reduce the number of SPI transfers in
about 25%.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>