arm: tegra: xmm: fix for kernel panic on xmm on/off

when baseband_xmm_power_switch is called or enumeration did not complete properly,
there is kernel panic because of extra unregister-ehci device
so set to null in uregister
https://github.com/riverzhou/kernel-tegra3/commit/8bed6fd166fb7221b603f7d79a0a70df4d7c6cc3