usb: host: ehci-msm: Use posted data writes on AHB

This patch sets the AHBMODE to allow for posted data writes.  This
results in higher performance.

Signed-off-by: Andy Gross <agross@codeaurora.org>
Tested-by: Georgi Djakov <georgi.djakov@linaro.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>