broadcom: Refine expansion register access routine

This patch makes the expansion register access routines a little more
formal.  They will be used by the following bcm50610 support patch.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>