Remove rhine_check_duplex, rhine_timer and related data structures

Add rhine_check_media: wrapper for generic mii_check_media, sets duplex
bit in MAC

Add rhine_enable_linkmon, rhine_disable_linkmon to enable hardware link
status monitoring

Update mdio_read, mdio_write accordingly

Remove get_intr_status check in rhine_start_tx because we are not racing
anymore
Signed-off-by: Roger Luethi <rl@hellgate.ch>