pinctrl: sx150x: access the correct bits in the 4-bit regs of sx150[147]

The code assumes 8-bit or 16-bit width registers, but three of the
chips (sx1501/sx1504/sx1507) are 4-bit. So, try to handle 4-bit chips as
well, they leave the high part of each register unused.
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-sx150x.c

@@ -5,6 +5,7 @@
  * Copyright (c) 2010, Code Aurora Forum. All rights reserved.
  *
  * Driver for Semtech SX150X I2C GPIO Expanders
+ * The handling of the 4-bit chips (SX1501/SX1504/SX1507) is untested.
  *
  * Author: Gregory Bean <gbean@codeaurora.org>
  *
@@ -1088,7 +1089,7 @@ static int sx150x_regmap_reg_write(void *context, unsigned int reg,
 
 	val = sx150x_maybe_swizzle(pctl, reg, val);
 
-	n = width - 8;
+	n = (width - 1) & ~7;
 	do {
 		const u8 byte = (val >> n) & 0xff;