net: sungem_phy: Constify struct mii_phy_def

net: sungem_phy: Constify struct mii_phy_def

'struct mii_phy_def' are not modified in this driver.

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

While at it fix the checkpatch warning related to this patch (some missing
newlines and spaces around *)

On a x86_64, with allmodconfig:
Before:
======
  27709	    928	      0	  28637	   6fdd	drivers/net/sungem_phy.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  28157	    476	      0	  28633	   6fd9	drivers/net/sungem_phy.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/54c3b30930f80f4895e6fa2f4234714fdea4ef4e.1723033266.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>