usb: use linux/usb/ch9.h instead of usbdescriptors.h

Linux usb/ch9.h seems to have all the same information (and more)
as usbdescriptors.h so use the former instead of the later one.

As a consequense of this change USB_SPEED_* values don't correspond
directly to EHCI speed encoding anymore, I've added necessary
recoding in EHCI driver. Also there is no point to put speed into
pipe anymore so it's removed and a bunch of host drivers fixed to
look at usb_device->speed instead.

Old usbdescriptors.h included is not removed as it seems to be
used by old USB device code.

This makes usb.h and usbdevice.h incompatible. Fortunately the
only place that tries to include both are the old MUSB code and
it needs usb.h only for USB_DMA_MINALIGN used in aligned attribute
on musb_regs structure but this attribute seems to be unneeded
(old MUSB code doesn't support any DMA at all).

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>