net: qca_spi: Don't clear IFF_BROADCAST

Currently qcaspi_netdev_setup accidentally clears IFF_BROADCAST.
So fix this by keeping the flags from ether_setup.

Reported-by: Michael Heimpold <michael.heimpold@i2se.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 291ab06e (net: qualcomm: new Ethernet over SPI driver for QCA7000)
Signed-off-by: David S. Miller <davem@davemloft.net>