mfd: altera-sysmgr: Fix a mistake caused by resource_size conversion

The resource_size defines that:
	res->end - res->start + 1;
The origin original code is:
	sysmgr_config.max_register = res->end - res->start - 3;

So, the correct fix is that:
	sysmgr_config.max_register = resource_size(res) - 4;

Fixes: d12edf96 ("mfd: altera-sysmgr: Use resource_size function on resource object")
Signed-off-by: Kai Song <songkai01@inspur.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211006141926.6120-1-songkai01@inspur.com