net: qca_spi: Fix possible race during probe

Registering the netdev before setting the priv data is unsafe.
So fix this possible race by setting the priv data first.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: <stable@vger.kernel.org> # v3.18+
Fixes: 291ab06e (net: qualcomm: new Ethernet over SPI driver for QCA7000)
Signed-off-by: David S. Miller <davem@davemloft.net>