ioat2: clarify ring size limits

With the addition of ioat_max_alloc_order it is not clear what the
maximum allocation order is, so document that in the modinfo.  Also take
an opportunity to kill a stray semicolon.

Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>