mei: validate the message header only in first fragment.

RX message header is received in the first fragment of
the message and saved side and it is not modified after that,
we don't need to validate it upon each fragment.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>