configs: libretech: set SPI mode to 0

Kconfig defaults to mode 3 if CONFIG_SF_DEFAULT_MODE is not set.
It becomes an issue since meson_spifc does not support SPI_CPHA.
Needed after commit e2e95e5e ("spi: Update speed/mode on change").

Fixes: e2e95e5e ("spi: Update speed/mode on change")
Signed-off-by: Da Xue <da@libre.computer>
[narmstrong: reformated commit reference & added Fixes tag]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>