Unregister ipmi_pnp_driver if we can't find ipmi_si.

Unregister ipmi_pnp_driver if we can't find ipmi_si.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Merge pull request #61 from taoma-tm/ipmi_fix

Unregister ipmi_pnp_driver if we can't find ipmi_si.