mmc: wmt-sdmmc: remove unused variable in wmt_complete_data_request()

The variable 'mmc' is initialized but never used, so remove it.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Chris Ball <cjb@laptop.org>