spi: atmel: Let spi core handle validating transfer length

spi core will handle validating transfer length since commit 4d94bd21b333
"spi: core: Validate length of the transfers in message".
So remove the same checking in this driver.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>