[SCSI] fix OOPS due to clearing eh_action prior to aborting eh command

The eh_action semaphore in scsi_eh_send_command is cleared after a
command timeout.  The command is subsequently aborted and the abort
will try to call scsi_done() on it.  Unfortunately, the scsi_eh_done()
routine unconditinally completes the semaphore (which is now null).
Fix this race by makiong the scsi_eh_done() routine check that the
semaphore is non null before completing it (mirroring the ordinary
command done/timeout logic).

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>