xhci: Don't warn about zeroed bMaxBurst descriptor field.

The USB 3.0 specification says that the bMaxBurst field in the SuperSpeed
Endpoint Companion descriptor is supposed to indicate how many packets a
SS device can handle before it needs to wait for an explicit handshake
from the host controller.  A zero value means the device can only handle
one packet before it needs a handshake.  Remove a warning in the xHCI
driver that implies this is an invalid value.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>