xhci-mem: Use self.sysdev instead of self.controller device node

self.sysdev based device is required to use DMA allocate and de-allocate
APIs. Hence use self.sysdev based device instead of self.controller based
device.

Change-Id: I48581582afb1ab8c8b4fb455b0246ab58d0bc87e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>