mv643xx_eth: fix inconsistent lock semantics

Nicolas Pitre noted that mv643xx_eth_poll was incorrectly using
non-IRQ-safe locks while checking whether to wake up the netdevice's
transmit queue.  Convert the locking to *_irq() variants, since we
are running from softirq context where interrupts are enabled.

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