i2c: mux: reg: check return value after calling platform_get_resource()

In the Linux kernel, the following vulnerability has been resolved:

i2c: mux: reg: check return value after calling platform_get_resource()

It will cause null-ptr-deref in resource_size(), if platform_get_resource() returns NULL, move calling resource_size() after devm_ioremap_resource() that will check 'res' to avoid null-ptr-deref. And use devm_platform_get_and_ioremap_resource() to simplify code.