net: dsa: dsa_loop: use int type to store negative error codes

Change the 'ret' variable in dsa_loop_init() from unsigned int to int, as
it needs to store either negative error codes or zero returned by
mdio_driver_register().

Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but can be confusing.  Additionally, assigning negative error
codes to unsigned type may trigger a GCC warning when the -Wsign-conversion
flag is enabled.

No effect on runtime.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20250903123404.395946-1-rongqianfeng@vivo.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>