mfd: syscon: Re-use resource_size() to count max_register

Instead of open coded variant use resource_size() and replace
weird '- 3' to more understandable '- 4'.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/syscon.c

@@ -245,7 +245,7 @@ static int syscon_probe(struct platform_device *pdev)
 	if (!base)
 		return -ENOMEM;
 
-	syscon_config.max_register = res->end - res->start - 3;
+	syscon_config.max_register = resource_size(res) - 4;
 	if (pdata)
 		syscon_config.name = pdata->label;
 	syscon->regmap = devm_regmap_init_mmio(dev, base, &syscon_config);