IB/mthca: Fix modify QP error path

If the call to mthca_MODIFY_QP() failed, then mthca_modify_qp() would
still do some things it shouldn't, such as store away attributes for
special QPs.  Fix this, and simplify the code, by simply jumping to
the exit path if mthca_MODIFY_QP() fails.

Signed-off-by: Roland Dreier <rolandd@cisco.com>