qca_spi: Log expected signature in error case

Most of the users doesn't know the expected signature of the QCA700x.
So provide it within the error message. Btw use lowercase for hex as
in the rest of the driver.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>