net: qualcomm: move qcaspi_tx_cmd to qca_spi.c

The function qcaspi_tx_cmd() is only called from qca_spi.c. So we better
move it there.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>