wifi: rtw89: debugfs: specify buffer size allocated by devm_kazlloc() for reading

Some debufs entries need output buffer over default size 4K. Since reading
of many debugfs entries is to access IO, it costs time to dynamically
re-allocate larger buffer and access IO again and again. Add an option to
specify the size it needs.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250122060310.31976-5-pkshih@realtek.com