EHCI: Make core reset and port speed reading more generic.

[mmel]

Summary

Use driver settable callbacks for handling of:

• core post reset
• reading actual port speed

Typically, OTG enabled EHCI cores wants setting of USBMODE register,
but this register is not defined in EHCI specification and different
cores can have it on different offset.

Also, for cores with TT extension, actual port speed must be determinable.
But again, EHCI specification not covers this so this patch provides
function for two most common variant of speed bits layout.

[• hselasky]

Looks good. I'll get some people to test it.

[• hselasky]

Do you want me to commit the patch?

[mmel]

Nope, i will do it (and i must request to rename my phabricator account).
Thanks for review.