usb_dc_stm32: reuse PMA buffer when possible.

Previously, endpoint configuration would reserve memory in the packet
memory area which would never be reclaimed. After this patch, endpoints
will reuse previously allocated memory when possible. We still leak
memory when reconfiguration increases the max packet size for a given
endpoint number, but this fixes the common case.

Bug: https://github.com/zephyrproject-rtos/zephyr/issues/23178


Signed-off-by: Josh Gao <josh@jmgao.dev>