mmc: dw_mmc: fix sparse non static symbol warning

Fixes the following sparse warning:

drivers/mmc/host/dw_mmc-k3.c:116:1: warning:
 symbol 'dw_mci_k3_pmops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Chris Ball <chris@printf.net>