smsc95xx: Add comments to the registers definition

This chip is used by a lot of embedded devices and also by the Raspberry
Pi 1, 2 & 3 which were created to promote the study of computer
sciences. Students wanting to learn kernel / network device driver
programming through those devices can only rely on the Linux kernel
driver source to make their own.

This commit adds a lot of comments to the registers definition to expand
the register names.

Cc: Steve Glendinning <steve.glendinning@shawell.net>
Cc: Microchip Linux Driver Support <UNGLinuxDriver@microchip.com>
CC: David Miller <davem@davemloft.net>
Signed-off-by: Martin Wetterwald <martin@wetterwald.eu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Steve Glendinning <steve.glendinning@shawell.net>
Acked-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>