cxgb4: fix thermal configuration dependencies

With CONFIG_THERMAL=m, we get a build error:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.c: In function 'cxgb4_thermal_get_trip_type':
drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.c:48:11: error: 'struct adapter' has no member named 'ch_thermal'

Once that is fixed by using IS_ENABLED() checks, we get a link error
against the thermal subsystem when cxgb4 is built-in:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.o: In function `cxgb4_thermal_init':
cxgb4_thermal.c:(.text+0x180): undefined reference to `thermal_zone_device_register'
drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.o: In function `cxgb4_thermal_remove':
cxgb4_thermal.c:(.text+0x1e0): undefined reference to `thermal_zone_device_unregister'

Finally, since CONFIG_THERMAL can be =m, the Makefile fails to pick up the
extra file into built-in.a, and we get another link failure against the
cxgb4_thermal_init/cxgb4_thermal_remove files, so the Makefile has to
be adapted as well to work for both CONFIG_THERMAL=y and =m.

Fixes: b1871915 ("cxgb4: Add thermal zone support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>