phy.h: fix obvious errors in doc and kerneldoc content

1) note that gianfar_phy.c was removed years ago
 2) fix obvious copy and paste error in regular doc
 3) change regular doc into kerneldoc for phy_modes()

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>