3c59x: Specify window explicitly for access to windowed registers

Currently much of the code assumes that a specific window has been
selected, while a few functions save and restore the window.  This
makes it impossible to introduce fine-grained locking.

Make those assumptions explicit by introducing wrapper functions
to set the window and read/write a register.  Use these everywhere
except vortex_interrupt(), vortex_start_xmit() and vortex_rx().
These set the window just once, or not at all in the case of
vortex_rx() as it should always be called from vortex_interrupt().

Cache the current window in struct vortex_private to avoid
unnecessary hardware writes.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Arne Nordmark <nordmark@mech.kth.se> [against 2.6.32]
Signed-off-by: David S. Miller <davem@davemloft.net>