gpio/grgpio: fix deadlock in grgpio_irq_unmap()

As reported by Alexey Khoroshilov:

    grgpio_irq_unmap() code looks quite suspicious regarding usage of
    priv->bgc.lock spinlock.

    It locks the spinlock in line 310:

	spin_lock_irqsave(&priv->bgc.lock, flags);

    and then it can call grgpio_set_imask() in line 317:

	grgpio_set_imask(priv, i, 0);

    But grgpio_set_imask() unconditionally locks the spinlock by itself.

Fix this by moving the spinlock acquisition outside of
grgpio_set_imask().

Found by Linux Driver Verification project (linuxtesting.org).

Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>