usb: host: xhci-mem: allocate zeroed Scratchpad Buffer

commit 7480d912d549f414e0ce39331870899e89a5598c upstream.

According to xHCI ch4.20 Scratchpad Buffers, the Scratchpad
Buffer needs to be zeroed.

	...
	The following operations take place to allocate
       	Scratchpad Buffers to the xHC:
	...
		b. Software clears the Scratchpad Buffer to '0'

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>