mmc: core: Initialize SET_BLOCK_COUNT request fields

Some request fields are initialized just before request processing
for sanity purposes. This is done for command, data, and stop parts
of the request, but not for sbc (set block count) part. Add such
initialization for that part too.

Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>