e1000: correctly handle phy_ctrl reserved & self-clearing bits

Make phyreg_writeops responsible for actually writing their
respective phy registers, rather than rely on set_mdic() to
do it on their behalf.

The only current instance of phyreg_writeops is set_phy_ctrl();
modify it to write the register on its own, while also correctly
handling reserved and self-clearing bits.

have_autoneg() does not need to check for MII_CR_RESTART_AUTO_NEG,
since the only time the flag comes into play is during set_phy_ctrl(),
and, following this patch, never actually gets written to the phy
control register.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
