usb: ehci should use u16 for isochronous intervals

While most isochronous endpoints have short polling intervals, the
EHCI driver won't necessarily handle larger ones correctly.

This patch switches to use a "u16" to represent those periods, not
a u8, since it can always work:  the largest expressible period
is 2^15 units ... not the previous too-short limit of 128 frames
(full or low speeds) or microframes (high speed, 32 frames).

This bug is essentially theoretical, since the few ISO endpoints
I've seen which don't use one transfer per frame are high speed
ones using more than that (including high bandwidth, 24 KB/msec).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>