drivers: mm: rat: Simplify sys_mm_drv_page_phys_get logic

Instead of tracking if a matching address has been found and then
doing the translation after, we can loop until one is found and
do the translation and return immediately.

This saves having to keep a "found" variable, and makes the code
shorter and more readable IMHO. Do this here.

Signed-off-by: Andrew Davis <afd@ti.com>