EHCI: don't try to clear the IAAD bit

This patch (as1541) corrects a small mistake in ehci-hcd.  The IAAD
(Interrupt on Async Advance Doorbell) bit in the USBCMD register is
designed, as its name says, to act as a "doorbell".  That is, the
driver activates the bit by setting it to 1, and the hardware
deactivates it later by setting it back to 0.  The driver cannot clear
the bit by writing a 0 to it; such writes are simply ignored.

Therefore there is no reason for ehci-hcd to try to clear the bit.
The patch removes the two instances where such attempts occur.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>