New PG2 firmwares have additional Rx descriptors.

Add a module parameter to manually set the number of Rx descriptors for
older versions (PG1). We cannot discriminate based on chip-id, since
this value must be set on probe.
Signed-off-by: NArik Nemtsov <arik@wizery.com>
Signed-off-by: NLuciano Coelho <coelho@ti.com>