mISDN: Remove VLAs

There's an ongoing effort to remove VLAs[1] from the kernel to eventually
turn on -Wvla. Remove the VLAs from the mISDN code by switching to using
kstrdup in one place and using an upper bound in another.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>