usb_hub_descriptor has to be packed as it's used for
communication with the device. Member wHubCharacteristics
violates the natural alignment rules.

Use explicit unaligned access functions for this member.
Fixes ARMv7 traping while using USB.

v2: fix typo found by Thomas Langer

v3: rebased on top of u-boot-usb/master

Signed-off-by: Lucas Stach <dev@lynxeye.de>