misdn: avoid -Wempty-body warning

gcc warns about a pointless condition:

drivers/isdn/hardware/mISDN/hfcmulti.c: In function 'hfcmulti_interrupt':
drivers/isdn/hardware/mISDN/hfcmulti.c:2752:17: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
 2752 |                 ; /* external IRQ */

As the check has no effect, just remove it.

Suggested-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>