isdn: isdnloop: fix logic error in isdnloop_sendbuf

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>