mmc: core: Don't handle buffers on stack while fetching CXD registers

Due to previous patches, all callers of mmc_send_cxd_data() now
allocates their buffers from the heap. This enables us to simplify
mmc_send_cxd_data() by removing the support of handling buffers, which
are allocated from the stack.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>