net: ethernet: microchip: lan743x: Fix skb allocation failure

The driver allocates skb during ndo_open with GFP_ATOMIC which has high chance of failure when there are multiple instances.
GFP_KERNEL is enough while open and use GFP_ATOMIC only from interrupt context.

Fixes: 23f0703c

 ("lan743x: Add main source files for new lan743x driver")
Signed-off-by: Yuiko Oshino <yuiko.oshino@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>