[TG3]: Add various register methods

This patch adds various dedicated register read/write methods for the
existing workarounds, including PCIX target workaround, write with read
flush, etc. The chips that require these workarounds will use these
dedicated access functions.

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