mmc: dw_mmc: Remove prev_state and state assignment for STATE_SENDING_CMD

Clang reports a compile warning:
drivers/mmc/host/dw_mmc.c:2124:5: warning: Value stored to 'prev_state'
is never read

By checking the code, prev_state and state assignment for
STATE_SENDING_CMD is indeed never used after jumping to unlock tag.
So remove it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>