net: qca_spi: clear IFF_TX_SKB_SHARING

ether_setup sets IFF_TX_SKB_SHARING but this is not supported by
qca_spi as it modifies the skb on xmit.

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>