buspirate_spi: Tristate IOs when using using pullup=on

Avoid putting 3.3V on IO pins when pullup=on to avoid damage to 1.8V
chips.

Signed-off-by: David Hendricks <david.hendricks@gmail.com>
Change-Id: I9ac4c6b7a0079bb1022f2d70030a6eb29996108f
Reviewed-on: https://review.coreboot.org/23864
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed