firmware: microchip: move buffer allocation into mpfs_auto_update_set_image_address()

This buffer is used exclusively by mpfs_auto_update_set_image_address(),
so move the management of it there, employing the recently added cleanup
infrastructure to avoid littering the function with gotos.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>