net: qed: make 'qed_ll2_ops_pass' as __maybe_unused

gcc warns about unused const variables even in header files when
building with W=1:

In file included from include/linux/qed/qed_rdma_if.h:14,
from drivers/net/ethernet/qlogic/qed/qed_rdma.h:16,
from drivers/net/ethernet/qlogic/qed/qed_cxt.c:23:
include/linux/qed/qed_ll2_if.h:270:33: error: 'qed_ll2_ops_pass' defined but not used [-Werror=unused-const-variable=]
270 | static const struct qed_ll2_ops qed_ll2_ops_pass = {

This one is intentional, so mark it as __maybe_unused to it can be
included from a file that doesn't use this variable.

Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Tested-by: Simon Horman <[e-mail]> # build-tested
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/qed/qed_ll2_if.h

···
 int qed_ll2_alloc_if(struct qed_dev *);
 void qed_ll2_dealloc_if(struct qed_dev *);
 #else
-static const struct qed_ll2_ops qed_ll2_ops_pass = {
+static __maybe_unused const struct qed_ll2_ops qed_ll2_ops_pass = {
 	.start = NULL,
 	.stop = NULL,
 	.start_xmit = NULL,