generic: net: phy: psb6970: fix missing-prototypes warnings

generic: net: phy: psb6970: fix missing-prototypes warnings

Fix the following build warnings on 6.12 kernel:

drivers/net/phy/psb6970.c:426:12: error: no previous prototype for 'psb6970_init' [-Werror=missing-prototypes]
  426 | int __init psb6970_init(void)
      |            ^~~~~~~~~~~~
drivers/net/phy/psb6970.c:434:13: error: no previous prototype for 'psb6970_exit' [-Werror=missing-prototypes]
  434 | void __exit psb6970_exit(void)
      |             ^~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19174
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>