spi: npcm_pspi: Fix the wrong clock divider calculation

Fix the wrong clock divider calculation.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
diff --git a/drivers/spi/npcm_pspi.c b/drivers/spi/npcm_pspi.c
index 37bab70..eb14185 100644
--- a/drivers/spi/npcm_pspi.c
+++ b/drivers/spi/npcm_pspi.c
@@ -144,7 +144,7 @@
 	if (speed > priv->max_hz)
 		speed = priv->max_hz;
 
-	divisor = DIV_ROUND_CLOSEST(apb_clock, (2 * speed) - 1);
+	divisor = DIV_ROUND_CLOSEST(apb_clock, (2 * speed)) - 1;
 	if (divisor > MAX_DIV)
 		divisor = MAX_DIV;