cxl/features: Add check for no entries in cxl_feature_info

cxl EDAC calls cxl_feature_info() to get the feature information and
if the hardware has no Features support, cxlfs may be passed in as
NULL.

[ 51.957498] BUG: kernel NULL pointer dereference, address: 0000000000000008
[ 51.965571] #PF: supervisor read access in kernel mode
[ 51.971559] #PF: error_code(0x0000) - not-present page
[ 51.977542] PGD 17e4f6067 P4D 0
[ 51.981384] Oops: Oops: 0000 [#1] SMP NOPTI
[ 51.986300] CPU: 49 UID: 0 PID: 3782 Comm: systemd-udevd Not tainted 6.17.0dj
test+ #64 PREEMPT(voluntary)
[ 51.997355] Hardware name: <removed>
[ 52.009790] RIP: 0010:cxl_feature_info+0xa/0x80 [cxl_core]

Add a check for cxlfs before dereferencing it and return -EOPNOTSUPP if
there is no cxlfs created due to no hardware support.

Fixes: eb5dfcb9e36d ("cxl: Add support to handle user feature commands for set feature")
Reviewed-by: Davidlohr Bueso <[e-mail]>
Reviewed-by: Alison Schofield <[e-mail]>
Signed-off-by: Dave Jiang <[e-mail]>

drivers/cxl/core/features.c

···
 {
 	struct cxl_feat_entry *feat;
 
+	if (!cxlfs || !cxlfs->entries)
+		return ERR_PTR(-EOPNOTSUPP);
+
 	for (int i = 0; i < cxlfs->entries->num_features; i++) {
 		feat = &cxlfs->entries->ent[i];
 		if (uuid_equal(uuid, &feat->uuid))