misc: ipfix-export unformat u16 collector_port fix

Use %U and unformat_udp_port instead of %u for unformat() call for
u16 collector_port number in set_ipfix_exporter_command_fn() to
avoid corruption of other variables which can happen if unformat()
with %u is used with a 16-bit variable. This avoids crash due to
corrupted fib_index value.

Type: fix

Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>
Change-Id: Id54273fcc458a7f9c5aa4025aa91711f160c1c1a