firewire: core: fix multichannel IR with buffers larger than 2 GB

With a 32-bit i, computing i<<PAGE_SHIFT might result in
an overflow and in an eventual sign-extension.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>