net: mvpp2: do not sleep in set_rx_mode

This patch replaces GFP_KERNEL by GFP_ATOMIC to avoid sleeping in the
ndo_set_rx_mode() call which is called with BH disabled.

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>