bfd: fix desired_min_tx_nsec update

bfd: fix desired_min_tx_nsec update

Type: fix

Currently, the change in desired_min_tx value initiated locally
doesn't result in actual update of the transmission interval.

bfd_session_t structure has two fields for each of protocol's
local time parameters
        desired_min_tx, required_min_rx
In a case of a parameter update,
"config_" prefixed fields store new values sent to remote
        bs.config_desired_min_tx_nsec,
        bs.config_required_min_rx_nsec
Those prefixed "effective_", keep old values, still in charge,
until new ones are being negotiated between peers:
        bs.effective_desired_min_tx_nsec,
        bs.effective_required_min_rx_nsec

Currently, upon termination of the Poll Sequence (negotiation) only
        bfd_set_effective_required_min_rx()
is called to update effective RX value. TX value remains unchanged.

With this fix, add a call to
        bfd_set_effective_desired_min_tx()
to set new, acknowledged TX value in effect.

Signed-off-by: Alexander Skorichenko <askorichenko@netgate.com>
Change-Id: I80b6746533839c9572598f1ad9dabb33e621a525