cxl/fwctl: Fix __fortify_panic

cxl/fwctl: Fix __fortify_panic

Fix a runtime assertion in cxlctl_get_supported_features(). Fortify
complains that it is potentially overflowing the entries array per
__counted_by_le(num_entries). Quiet the false positive by initializing
@num_entries earlier.

 memcpy: detected buffer overflow: 48 byte write of buffer size 0
 WARNING: lib/string_helpers.c:1036 at __fortify_report+0x4d/0xa0, CPU#7: fwctl/1398
 RIP: 0010:__fortify_report+0x50/0xa0
 Call Trace:
  __fortify_panic+0xd/0xf
  cxlctl_get_supported_features.cold+0x23/0x35 [cxl_core]

Fixes: 4d1c09cef2c2 ("cxl: Add support for fwctl RPC command to enable CXL feature commands")
Signed-off-by: Dan Williams <djbw@kernel.org>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260519221204.1517773-2-djbw@kernel.org
Signed-off-by: Dave Jiang <dave.jiang@intel.com>