cxgb4: unexport cxgb4_dcb_enabled

A recent cleanup marked cxgb4_dcb_enabled as 'static', which is correct, but this ignored
how the symbol is also exported. In addition, the export can be compiled out when modules
are disabled, causing a harmless compiler warning in configurations for which it is not
used at all:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c:282:12: error: 'cxgb4_dcb_enabled' defined but not used [-Werror=unused-function]

This removes the export and moves the function into the correct #ifdef so we only build
it when there are users.

Fixes: 50935857 ("cxgb4: mark symbols static where possible")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>