mlx4_core: Don't leak mailbox for SET_PORT on Ethernet ports

Commit 793730bf ("mlx4_core: Don't perform SET_PORT command for
Ethernet ports") introduced a leak of mailbox buffers when SET_PORT
was called for Ethernet ports, since it added a return after the
mailbox was allocated.  Fix this by checking the port type and
returning *before* allocating the mailbox.

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