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>