MFC r361581:
Implement helper function, usbd_get_max_frame_length(), which allows kernel
device drivers to correctly predict the default USB transfer frame length.

Sponsored by: Mellanox Technologies

MFC r286773:
Improve the realtime properties of USB transfers for embedded systems
like RPI-B and RPI-2.

MFC r267240:
Resolve a deadlock setting the USB configuration index from userspace
on USB HUBs by moving the code into the USB explore threads. The
deadlock happens because child devices of the USB HUB don't have the
expected reference count when called from outside the explore
thread. Only the HUB device itself, which the IOCTL interface locks,
gets the correct reference count.

MFC r263159:
Workaround for USB MIDI adapters which use non-supported values of
wMaxPacketSize for BULK endpoints.

MFC r286773:
Improve the realtime properties of USB transfers for embedded systems
like RPI-B and RPI-2.

MFC r267240:
Resolve a deadlock setting the USB configuration index from userspace
on USB HUBs by moving the code into the USB explore threads. The
deadlock happens because child devices of the USB HUB don't have the
expected reference count when called from outside the explore
thread. Only the HUB device itself, which the IOCTL interface locks,
gets the correct reference count.

MFC r263159:
Workaround for USB MIDI adapters which use non-supported values of
wMaxPacketSize for BULK endpoints.