spi: nxp_fspi: Fix error reporting

On the i.MX8M Mini, ret = clk_set_rate() sets ret to the value of the
rate the clock was able to set.  When checking for errors, it only
checks that it is not NULL.  Since positive numbers are not errors,
only check for negative numbers when handling errors.

Fixes: 383fded7 ("spi: nxp_fspi: new driver for the FlexSPI controller")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>