firewire: cdev: address handler input validation

Like before my commit 1415d9189e8c59aa9c77a3bba419dcea062c145f,
fw_core_add_address_handler() does not align the address region now.
Instead the caller is required to pass valid parameters.

Since one of the callers of fw_core_add_address_handler() is the cdev
userspace interface, we now check for valid input.  If the client is
buggy, we give it a hint with -EINVAL.

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