octeontx2-pf: Fix unused variable build error

octeontx2-pf: Fix unused variable build error

If CONFIG_DCB is not set,
make ARCH=x86_64 CROSS_COMPILE=x86_64-linux-gnu-,
will be failed, like this:

drivers/net/ethernet/marvell/octeontx2/nic/otx2_pf.c: In function ‘otx2_select_queue’:
drivers/net/ethernet/marvell/octeontx2/nic/otx2_pf.c:1886:19: error: unused variable ‘pf’ [-Werror=unused-variable]
  struct otx2_nic *pf = netdev_priv(netdev);
                   ^~
cc1: all warnings being treated as errors

To fix this build error, put the definition of *pf under the CONFIG_DCB.

Fixes: e64a4898c6c6 ("octeontx2-pf: Add egress PFC support")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Link: https://lore.kernel.org/r/20220919025840.256411-1-renzhijie2@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>