usb: host: xhci-plat: Use xhci dev instead of dwc3 dev with probe

snps,usb3-lpm-capable property is need to use with dwc3 device node,
whereas usb3_lpm_capable property is used with xhci-plat device node.
snps,usb3-lpm-capable property value is updated with usb3_lpm_capable
property from dwc3_host_init() API. Hence trying to get usb3_lpm_capable
property with dwc3 device always returns zero irrespective of set value
with snps,usb3-lpm-capable property. Fix this issue by using xhci device
(i.e. &pdev->dev) instead of dwc3 device (i.e. sysdev).

Change-Id: I1f1540bdfeaa21f3f3309c1a4458b83971d2a438
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>