net: qca_spi: Avoid packet drop during initial sync

As long as the synchronization with the QCA7000 isn't finished, we
cannot accept packets from the upper layers. So let the SPI thread
enable the TX queue after sync and avoid unwanted packet drop.

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>