spi.c: Fix logic errors in write_dfifo function

Change-Id: I25867012e644b825f0a352dacb20e9df3a0c0720
Reviewed-on: https://review.coreboot.org/c/em100/+/42571
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
1 file changed