pcmcia: Fix requery

The requery logic goes off and attempts to read the CIS of empty slots. In
most cases this happens not to do any harm - but not all!

Add the missing check and also a WARN() to catch any other offenders.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>