wifi: mt76: Drop unneeded mt76_register_debugfs_fops() return checks

wifi: mt76: Drop unneeded mt76_register_debugfs_fops() return checks

mt76_register_debugfs_fops() returns the dentry from
debugfs_create_dir(), which yields an error pointer on failure
(notably ERR_PTR(-ENODEV) when CONFIG_DEBUG_FS=n), never NULL. Per
commit ff9fb72bc077 ("debugfs: return error values, not NULL"),
callers do not need to check the return value.

Drop the dead !dir checks in mt7615/mt7915/mt7921/mt7925/mt7996
_init_debugfs(). Converting them to IS_ERR() instead would have
exposed a probe abort on CONFIG_DEBUG_FS=n, since each
*_init_debugfs() caller propagates the helper's return value.

This patch supersedes an earlier proposal that converted the checks
to IS_ERR().

Link: https://lore.kernel.org/all/20260514193243.2518979-1-ingyujang25@korea.ac.kr
Signed-off-by: Ingyu Jang <ingyujang25@korea.ac.kr>
Link: https://patch.msgid.link/20260519085214.164846-1-ingyujang25@korea.ac.kr
Signed-off-by: Felix Fietkau <nbd@nbd.name>