xhci: Check size rather then number of streams when allocating stream ctxs

Before this a device needing ie 32 stream ctxs would end up with an entry from
the small_streams_pool which has 256 bytes entries, where as 32 stream ctxs
need 512 bytes. Things actually keep running for a surprisingly long time
before crashing because of this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>