mmc: mmci: add get_next_data callback

This patch adds get_next_data callback to mmci_host_ops.
Generic mmci_get_next_data factorizes next_cookie check and
the host ops call.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>