mctp: Fix check for dev_hard_header() result

dev_hard_header() returns the length of the header, so
we need to test for negative errors rather than non-zero.

Fixes: 889b7da2 ("mctp: Add initial routing framework")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>