spl: spi: fix falcon mode for spi boot

spl_start_uboot is a board overridable function that switches to falcon
boot mode on return value of 0.

Though for SPI, the falcon boot mode was being enabled on return value
of 1 which is not the correct behaviour. Therefore this patch fixes it
to the expected boot flow.

Fixes: 14509a28aa20 ("spl: spi: Consolidate spi_load_image_os into spl_spi_load_image")
Signed-off-by: Anshul Dalal <anshuld@ti.com>