spi: atmel: fix incorrect comparison

Found using smatch:
drivers/spi/spi-atmel.c:878 atmel_spi_pump_pio_data() warn: unsigned
'as->current_remaining_bytes' is never less than zero.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>