hwmon: (smsc47m1) fix (suspicious) outside array bounds warnings

Kbuild test robot reports outside array bounds warnings.

This is reproducible for ARCH=sh allmodconfig with the kernel.org
toolchains available at:

https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/8.1.0/x86_64-gcc-8.1.0-nolibc-sh4-linux.tar.xz

  CC [M]  drivers/hwmon/smsc47m1.o
drivers/hwmon/smsc47m1.c: In function 'fan_div_store':
drivers/hwmon/smsc47m1.c:370:49: warning: array subscript [0, 2] is outside array bounds of 'u8[3]' {aka 'unsigned char[3]'} [-Warray-bounds]
  tmp = 192 - (old_div * (192 - data->fan_preload[nr])
                                ~~~~~~~~~~~~~~~~~^~~~
drivers/hwmon/smsc47m1.c:372:19: warning: array subscript [0, 2] is outside array bounds of 'u8[3]' {aka 'unsigned char[3]'} [-Warray-bounds]
  data->fan_preload[nr] = clamp_val(tmp, 0, 191);
  ~~~~~~~~~~~~~~~~~^~~~
drivers/hwmon/smsc47m1.c:373:53: warning: array subscript [0, 2] is outside array bounds of 'const u8[3]' {aka 'const unsigned char[3]'} [-Warray-bounds]
  smsc47m1_write_value(data, SMSC47M1_REG_FAN_PRELOAD[nr],
                             ~~~~~~~~~~~~~~~~~~~~~~~~^~~~

Looking at the code, I believe these are false positives.

While it is ridiculous to patch our driver to make the insane
compiler happy, clarifying the unreachable path will be helpful
not only for compilers but also for humans.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
[groeck: Use BUG() instead of unreachable() to make objtool happy]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/smsc47m1.c

@@ -351,6 +351,8 @@ static ssize_t fan_div_store(struct device *dev,
 		tmp |= data->fan_div[2] << 4;
 		smsc47m1_write_value(data, SMSC47M2_REG_FANDIV3, tmp);
 		break;
+	default:
+		BUG();
 	}
 
 	/* Preserve fan min */