dcache: Remove dependency of r1.wb.adr/dat/sel on req_op

dcache:  Remove dependency of r1.wb.adr/dat/sel on req_op

This improves timing by setting r1.wb.{adr,dat,sel} to the next
request when doing a write cycle on the wishbone before we know
whether the next request has a TLB and cache hit or not, i.e.
without depending on req_op.  r1.wb.stb still depends on req_op.

This contains a workaround for what is probably a bug elsewhere,
in that changing r1.wb.sel unconditionally once we see stall=0
from the wishbone causes incorrect behaviour.  Making it
conditional on there being a valid following request appears
to fix the problem.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>