ramips: remove redundant status for USB controllers

ramips: remove redundant status for USB controllers

mt7621, mt7628an and rt5350 have USB controllers (ehci/ohci or xhci)
enabled by default. Thus, this patch drops redundant status=okay
statements in derived device DTS files.

While at it, also drop an explicit status=okay in mt7621.dtsi, as
this is default.

Note:

For rt5350, about 50 % of the devices enabled ehci/ohci in the DTS
files, and there is actually no device actively disabling it.
It looks like only a few people are aware that the controllers are
enabled by default here.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>