Disable USB PHY slew rate calibration for Mediatek SoCs for now

USB on both MT7621 and MT7688 seems to work much better without doing
slew rate calibration.
These are the only two SoCs, apart from MT7628, which actually make
use of the slew rate calibration routines implemented in the mtk_usb_phy
driver. Since MT7628 is actually a superset of MT7688 things should be
the same for it as well.

We do not remove the code, we simply define it out.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5884