wifi: mt7601u: delete dead code checking debugfs returns

Smatch reports that:
drivers/net/wireless/mediatek/mt7601u/debugfs.c:130
mt7601u_init_debugfs() warn: 'dir' is an error pointer or valid".

Debugfs code is not supposed to need error checking so instead of
changing this to if (IS_ERR()) the correct thing is to just delete
the dead code.

Signed-off-by: Wang Jikai <wangjikai@hust.edu.cn>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230421092200.24456-1-wangjikai@hust.edu.cn