net: stmmac: rk: use rk_encode_wm16() for RMII speed

net: stmmac: rk: use rk_encode_wm16() for RMII speed

The RMII speed configuration is encoded as a single bit, which is set
for 100M and clean for 10M. Provide the bitfield definition in
struct rk_clock_fields, moving it out of struct rk_reg_speed_data's
rmii_10 and rmii_100 initialisers. Update rk_set_reg_speed() to handle
the new definition location of this bit.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vmqnC-00000007VCt-0oRg@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>