spi/tegra20-slink: Remove unused is_single_xfer check

Currently transfer_one_message() checks to see if the message consists of
a single spi_transfer and tells _start_transfer_one() but it just ignores
this. Don't bother.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>