spi_nxp_lpspi: Fix underflow in lpspi sck calculation

If `desired_delay_ns` is `0`, it is much saner to set the delay to `0x00`
than to underflow and set it to `0xff`, which is the current behavior.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>
(cherry picked from commit 860f9c8e)