mtd: spi: don't return -1 when scan succeed

mtd: spi: don't return -1 when scan succeed

In spi_flash_scan, 'ret' is initialled to -1, but 'ret' is not always
used to store a return value, in that case, even when the function
succeed, an error (-1) will be returned.
Lets just return 0 if we hit the end of the function.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>