wifi: rtw89: debugfs: use debugfs_short_fops

With this change, the object code size can reduce 768 bytes.

   text    data     bss     dec     hex filename
  77257    4262       4   81523   13e73 debug.o   (before)
  76489    4262       4   80755   13b73 debug.o   (after)

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250122060310.31976-7-pkshih@realtek.com