dnet: mark methods static and annotate for correct endianness

Their doesn't appear to be bugs with the endianness handling here, just get the
annotations right to keep sparse happy.

Suppresses the following sparse warnings:
drivers/net/dnet.c:30:5: warning: symbol 'dnet_readw_mac' was not declared. Should it be static?
drivers/net/dnet.c:49:6: warning: symbol 'dnet_writew_mac' was not declared. Should it be static?
drivers/net/dnet.c:364:5: warning: symbol 'dnet_phy_marvell_fixup' was not declared. Should it be static?
drivers/net/dnet.c:66:13: warning: incorrect type in assignment (different base types)
drivers/net/dnet.c:66:13:    expected unsigned short [unsigned] [usertype] tmp
drivers/net/dnet.c:66:13:    got restricted __be16 [usertype] <noident>
drivers/net/dnet.c:68:13: warning: incorrect type in assignment (different base types)
drivers/net/dnet.c:68:13:    expected unsigned short [unsigned] [usertype] tmp
drivers/net/dnet.c:68:13:    got restricted __be16 [usertype] <noident>
drivers/net/dnet.c:70:13: warning: incorrect type in assignment (different base types)
drivers/net/dnet.c:70:13:    expected unsigned short [unsigned] [usertype] tmp
drivers/net/dnet.c:70:13:    got restricted __be16 [usertype] <noident>
drivers/net/dnet.c:92:27: warning: cast to restricted __be16
drivers/net/dnet.c:94:33: warning: cast to restricted __be16
drivers/net/dnet.c:96:33: warning: cast to restricted __be16

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>