mmc: mxcmmc: check the return value of mxcmci_finish_data

We got a compile warning for mxcmmc,

drivers/mmc/host/mxcmmc.c: In function 'mxcmci_data_done':
drivers/mmc/host/mxcmmc.c:661:6: warning: variable 'data_error' set but
not used [-Wunused-but-set-variable]

The easiest method is to remove the data_error. But looking into
the code closely, I think we should check the return value of
mxcmci_finish_data as if it got data->error(the same as data_error),
we shouldn't try to read the response.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>