mm_drv: tlb: Fix driver tests

Previous fix https://github.com/zephyrproject-rtos/zephyr/pull/58891


introduced failure in driver tests suite. This patch corrects the error
and reverts max_mapped_page field definition.
Adds max_mapped_pages stats reset after unmaping of unused memory.

Signed-off-by: Jaroslaw Stelter <Jaroslaw.Stelter@intel.com>