Bluetooth: btusb: Fix not handling ZPL/short-transfer

Requesting transfers of the exact same size of wMaxPacketSize may result
in ZPL/short-transfer since the USB stack cannot handle it as we are
limiting the buffer size to be the same as wMaxPacketSize.

Also, in terms of throughput this change has the same effect to
interrupt endpoint as 290ba200 "Bluetooth: Improve USB driver throughput
by increasing the frame size" had for the bulk endpoint, so users of the
advertisement bearer (e.g. BT Mesh) may benefit from this change.

Fixes: 5e23b923 ("[Bluetooth] Add generic driver for Bluetooth USB devices")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Kiran K <kiran.k@intel.com>