cnic: Add a signature to indicate valid doorbell offset.

The buffer that is used to pass doorbell offset to the userspace UIO
driver may contain nonzero value in older versions of bnx2x driver.
Userspace cannot easily tell whether it contains a valid doorbell
offset or not.  With the added signature, userspace will only use
the doorbell offset if the signature is present.

Update version to 2.5.19.

Signed-off-by: Eddie Wai <eddie.wai@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>