usb: xhci-plat: Add XHCI_STATE_REMOVING flag on removal of usb device

Commit 5dcd26f4e903 ("xhci: fix 10 second timeout on removal of PCI
hotpluggable xhci controllers") avoids queueing configure_endpoint
commands for the dropped endpoints.

This change adds XHCI_STATE_REMOVING flag on removal of usb device.
For usb XHCI hotplugged controllers this will prevent 5 second command
timeouts and avoid the races with mutex lock.

CRs-Fixed: 1031459
Change-Id: Ic953c4ea17562357cb3845b77c0ab08b7daa1a4d
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>