bugfix: loop for ortgmem_type should be for the number of MAX_RTG_BOARDS

bugfix: loop for ortgmem_type should be for the number of MAX_RTG_BOARDS

ortgmem_type is defined having a size of MAX_RTG_BOARDS, but the loop uses MAX_RAM_BOARDS when going through it.

Since MAX_RAM_BOARDS and MAX_RTG_BOARDS happen to be the same (4) in WinUAE, it didn't have any effect. However, if those happen to be different (like in Amiberry, where MAX_RTG_BOARDS is set to 1), then this will cause undefined behavior after the first iteration.