wlcore: might_sleep operation in interrupt handler

Interrupt handler wlcore_irq in wlcore/main.c file
invokes mutex_lock that is might sleep operation.

Found by Linux Driver Verification project (linuxtesting.org)
within my participation in the GSoC 2012 program.

drivers/net/wireless/ti/wlcore/main.c

static irqreturn_t wlcore_irq(int irq, void *cookie)
{
        int ret;
        unsigned long flags;
        struct wl1271 *wl = cookie;

        /* TX might be handled here, avoid redundant work */
        set_bit(WL1271_FLAG_TX_PENDING, &wl->flags);
        cancel_work_sync(&wl->tx_work);

        mutex_lock(&wl->mutex);
...
}

int __devinit wlcore_probe(struct wl1271 *wl, struct platform_device *pdev)
{
...
        ret = request_threaded_irq(wl->irq, wl12xx_hardirq, wlcore_irq,
                                   irqflags,
                                   pdev->name, wl);
...
}