mv643xx_eth: switch to netif tx queue lock, get rid of private spinlock

Since our ->hard_start_xmit() method is already called under spinlock
protection (the netif tx queue lock), we can simply make that lock
cover the private transmit state (descriptor ring indexes et al.) as
well, which avoids having to use a private lock to protect that state.

Since this was the last user of the driver-private spinlock, it can
be killed off.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>