CC: brcm63xx: ensure dummy byte is set for mapped spi flash with fast read

CC: brcm63xx: ensure dummy byte is set for mapped spi flash with fast read
Some CFEs seem to misconfigure the mapped memory flash access with
fast read but without a dummy byte, causing all accesses to be prefixed
with 0xff.
This of course breaks reading out the nvram, so do not just move back to
single i/o accessors, but also ensure that the dummy byte is correctly
set.

Backport of r46707.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46709 3c298f89-4303-0410-b956-a3cf2f4a3e73