isdn: isdnloop: fix logic error in isdnloop_sendbuf

gcc-7 found an ancient bug in the loop driver, leading to a condition that
is always false, meaning we ignore the contents of 'card->flags' here:

drivers/isdn/isdnloop/isdnloop.c:412:37: error: ?: using integer constants in boolean context, the expression will always evaluate to 'true' [-Werror=int-in-bool-context]

This changes the braces in the expression to ensure we actually
compare the flag bits, rather than comparing a constant. As Joe Perches
pointed out, an earlier patch of mine incorrectly assumed this was a
false-positive warning.

Cc: Joe Perches <joe@perches.com>
Link: https://patchwork.kernel.org/patch/9840289/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/isdnloop/isdnloop.c

@@ -409,7 +409,7 @@ isdnloop_sendbuf(int channel, struct sk_buff *skb, isdnloop_card *card)
 		return -EINVAL;
 	}
 	if (len) {
-		if (!(card->flags & (channel) ? ISDNLOOP_FLAGS_B2ACTIVE : ISDNLOOP_FLAGS_B1ACTIVE))
+		if (!(card->flags & (channel ? ISDNLOOP_FLAGS_B2ACTIVE : ISDNLOOP_FLAGS_B1ACTIVE)))
 			return 0;
 		if (card->sndcount[channel] > ISDNLOOP_MAX_SQUEUE)
 			return 0;