mptcp: Constify mptcp_pm_ops

The only usages of mptcp_pm_ops is to assign its address to the small_ops
field of the genl_family struct, which is a const pointer, and applying
ARRAY_SIZE() on it. Make it const to allow the compiler to put it in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>