ar71xx: Change led trigger from usbdev to usbport

ar71xx: Change led trigger from usbdev to usbport

This patch changes all the legacy usbdev leds triggers
to the use usbport. The problem is that the usbdev triggers
get applied as usbport in sysfs but LuCI gets confused and
is not able to show the configuration with usbdev. More
information can be found in the PR:
<https://github.com/openwrt/openwrt/pull/1819>

Signed-off-by: David Santamaría Rogado <howl.nsp@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
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