mtd: spi-nor: Fix an error code in spi_nor_read_raw()

The problem is that if "ret" is negative then when we check if
"ret > len", that condition is going to be true because of type
promotion.  So this patch re-orders the code to check for negatives
first and preserve those error codes.

Fixes: f384b352 ("mtd: spi-nor: parse Serial Flash Discoverable Parameters (SFDP) tables")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>