This adds a sanity check in the interrupt routine
insures incoming message frames are a valid
message frames.

The code for setting 0xdeadbeaf in the freed message
frames, apparently was already submitted by Christoph
in previous patch submission.
Signed-off-by: Eric Moore <Eric.Moore@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>