net: phy: Make use of linkmode_mod_bit helper

linkmode_mod_bit is introduced as a helper function to set/clear
bits in a linkmode.
Replace the if else code structure with a call to the helper
linkmode_mod_bit.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>