spi: davinci: Remove unneeded NULL checking for dspi and dspi->bitbang.master

spi_master_get_devdata() never returns NULL when spi_alloc_master() success,
so remove NULL test for dspi.
We have ensured master is not NULL before assigning it to dspi->bitbang.master.
So also remove NULL test for dspi->bitbang.master.

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