isdn: fix a wrapping bug in isdn_ppp_ioctl()

"protos" is an array of unsigned longs and "i" is the number of bits in
an unsigned long so we need to use 1UL as well to prevent the shift
from wrapping around.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>