board: traverse: ten64: ensure retimer reset is done on new board revisions

Board revision C (production) and later require the SFP+
retimer to be turned on (or reset) on boot, by way of issuing
a command to the board's microcontroller (via I2C).

The comparison statement here was incorrect, as the board
ID decrements every revision (from 0xFF downwards),
so this was matching board RevA,B,C instead of Rev >= C.

Another oops that transpired when working on this issue,
is that if the board controller is not called (such as
CONFIG_TEN64_CONTROLLER=n or earlier board rev), then
the retimer udevice was not obtained. So the board
version check has to be moved inside board_cycle_retimer
(which probes/fetches the retimer device) as well.

Signed-off-by: Mathew McBride <matt@traverse.com.au>