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.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 46707