spi: loopback-test: don't skip comparing the first byte of rx_buf

When the loopback parameter is set, rx_buf are compared with tx_buf
after the spi_message is executed.  But the first byte of buffer is
not checked.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>