arm: tegra: xmm: device set to null in unregister

arm: tegra: xmm: device set to null in unregister

hsic_unregister is being called multiple times
during two step enumeration process of xmm.
In failure cases when enumeration does not
complete properly, kernel panic is seen because
of extra unregister-ehci device.

Bug 1016593

Change-Id: Ibaeb20ed0dab0e906741fdfad78f7d89a3672f60
Signed-off-by: Vinayak Pane <vpane@nvidia.com>
Reviewed-on: http://git-master/r/116626
Reviewed-by: Simone Willett <swillett@nvidia.com>
Tested-by: Simone Willett <swillett@nvidia.com>
Rebase-Id: R459c0adfc70ecc1b5ae3ea72615524f25e96dc55