cxgb4: fix thermal zone build error

with CONFIG_THERMAL=m and cxgb4 as built-in build fails, and
'commit e70a57fa ("cxgb4: fix thermal configuration dependencies")'
tries to fix it but when cxgb4i is made built-in build fails again,
use IS_REACHABLE instead of IS_ENABLED to fix the issue.

Fixes: e70a57fa (cxgb4: fix thermal configuration dependencies)
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>