mctp: Fix incorrect tx flow invalidation condition in mctp-i2c

Previously, the condition for invalidating the tx flow in
mctp_i2c_invalidate_tx_flow() checked if `rc` was nonzero.
However, this could incorrectly trigger the invalidation
even when `rc > 0` was returned as a success status.

This patch updates the condition to explicitly check for `rc < 0`,
ensuring that only error cases trigger the invalidation.

Signed-off-by: Daniel Hsu <Daniel-Hsu@quantatw.com>
Reviewed-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>