net: phy: Constify struct mdio_device_id

'struct mdio_device_id' is not modified in these drivers.

Constifying these structures moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  27014	  12792	      0	  39806	   9b7e	drivers/net/phy/broadcom.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  27206	  12600	      0	  39806	   9b7e	drivers/net/phy/broadcom.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/403c381b7d9156b67ad68ffc44b8eee70c5e86a9.1736691226.git.christophe.jaillet@wanadoo.fr


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