sky2: Refactor sky2_get_regs into two functions

Separate code deciding which registers can be accessed out of
sky2_get_regs in preparation for adding more conditions into it.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>