spi: Dont call prepare/unprepare transfer if not populated

Currently the prepare/unprepare transfer are called unconditionally.
The assumption is that every driver using the spi core queue infrastructure
has to populate the prepare and unprepare functions. This encourages
drivers to populate empty functions to prevent crashing.
This patch prevents the call to prepare/unprepare if not populated.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
[grant.likely: fix whitespace defect]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>