mISDN: use memchr_inv

Use memchr_inv to check if the data contains all same bytes.  It is
faster than looping for each byte.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>