pds_core: fix debugfs_lookup dentry leak and error handling

debugfs_lookup() returns a dentry with an elevated reference count that
must be released with dput(). The current code discards the returned
dentry without calling dput(), causing a reference leak on every
firmware reset recovery.

Additionally, when CONFIG_DEBUG_FS is disabled, debugfs_lookup()
returns ERR_PTR(-ENODEV), not NULL. The current check passes for error
pointers and would call dput() on an invalid pointer, causing a crash.

Fixes: bc90fbe0c318 ("pds_core: Rework teardown/setup flow to be more common")
Signed-off-by: Nikhil P. Rao <nikhil.rao@amd.com>
Link: https://patch.msgid.link/20260515212907.998028-3-nikhil.rao@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>