Add a comment about the MPSAFEness of this pccard_intr handler. Given

how we registered pccard_intr, it is MPSAFE.  However, since we
register the pccard_intr handler with the flags of the ISR we call,
that is the gating factor.  We need do nothing specific here.

Prompted by: seeing pccard_intr in a panic.

sys/dev/pccard/pccard.c

@@ -1202,6 +1202,10 @@ pccard_intr(void *arg)
 	 * the interrupt will pacify the card enough to keep an
 	 * interrupt storm from happening.  Of course this won't
 	 * help in the non-MFC case.
+	 *
+	 * This has no impact for MPSAFEness of the client drivers.
+	 * We register this with whatever flags the intr_handler
+	 * was registered with.  All these functions are MPSAFE.
 	 */
 	if (pccard_mfc(pf->sc)) {
 		reg = pccard_ccr_read(pf, PCCARD_CCR_STATUS);