spi: tegra20_slink: accept any word length

Original t20 slink could work with commands only
fully divisible by 8. This patch removes such
restriction, so commands of any bitlength now
can be passed and processed.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS TF600T T30
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # LG P895 T30
Tested-by: Thierry Reding <treding@nvidia.com> # T30 and T124
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Signed-off-by: Tom <twarren@nvidia.com>