board: ten64: add missing error checks for retimer power on

The retimer reset/power on logic was changed in a recent commit,
however, it neglected to check if the commands sent to the
board microcontroller (to control power to the retimer chip)
actually completed.

Add return checks for these operations so any failures will
be reported to the user.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Fixes: 7a041fea ("board: traverse: ten64: ensure retimer reset
is done on new board revisions")