realtek: mdio-serdes: improve debugfs creation

realtek: mdio-serdes: improve debugfs creation

Commit 3c073b5cb2 cleaned up the debugfs creation in
mdio-realtek-otto-serdes driver to not explicitly check if the root
directory already exists. This is fine because kernel handles the case
properly so there's no need to check anymore.

However, this pollutes the boot log with:
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'
[..] debugfs: 'realtek_otto_serdes' already exists in '/'

Now, the root directory creation is attempted multiple times, causing
the kernel to print an error message because the directory already
exists.

Fix this by moving the SerDes loop into rtsds_debug_init and only try
to create the root debugfs directory once.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21179
Signed-off-by: Robert Marko <[email protected]>
(cherry picked from commit 8e4730fd60a1b4998d1c9c07aeb6c6a27bce210f)