IB/qib: Add missing serdes init sequence

commit 2f75e12c4457a9b3d042c0a0d748fa198dc2ffaf upstream.

Research has shown that commit a77fcf895046 ("IB/qib: Use a single
txselect module parameter for serdes tuning") missed a key serdes init
sequence.

This patch add that sequence.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>