usb_signalling: Track USB symbol width to compensate frequency errors

USB low/full speed allows for frequency tolerance of 1.5%/0.25%. At
maximum packet size (sync + PID + data + CRC16) of 12 bytes/1027 bytes
this amounts to 1.4 bits/20 bits, so the decoder has to lock to the
actual symbol frequency to avoid any symbol misdetections.

The signal is sampled twice, once at the symbol center and once at
the expected edge position. Comparing the symbol at both positions gives
an indication if the current bit width is too low or too high. Adjust
accordingly.

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