Correct handling of shared interrupt in sis_intr(). r212116 incorrectly released a drver lock for shared interrupt case such that it caused panic. While I'm here check whether driver is still running before serving TX/RX handler.
Correct handling of shared interrupt in sis_intr(). r212116 incorrectly
released a drver lock for shared interrupt case such that it caused
panic. While I'm here check whether driver is still running before
serving TX/RX handler.

Reported by:	Jerahmy Pocott < QUAKENET1 <> optusnet dot com dot au >
Tested by:	Jerahmy Pocott < QUAKENET1 <> optusnet dot com dot au >
MFC after:	3 days

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