mfd: pm8xxx-pwm: select higher resolution if possible

More bits in PWM size means higher resolution, which is preferred
if we have such choice without sacrificing accuracy.

Signed-off-by: Willie Ruan <wruan@codeaurora.org>
diff --git a/drivers/mfd/pm8xxx-pwm.c b/drivers/mfd/pm8xxx-pwm.c
index cdddd98..a6baa2a 100644
--- a/drivers/mfd/pm8xxx-pwm.c
+++ b/drivers/mfd/pm8xxx-pwm.c
@@ -340,6 +340,12 @@
 		}
 	}
 
+	/* Use higher resolution */
+	if (best_m >= 3 && n == 6) {
+		n += 3;
+		best_m -= 3;
+	}
+
 	pwm_conf->pwm_size = n;
 	pwm_conf->clk = best_clk;
 	pwm_conf->pre_div = best_div;