mmc: sh_mmcif: remove redundant initialization of 'opc'

Variable opc is initialized with a value that is never read, opc
is later re-assigned a newer value, hence the initialization can
be removed.

Cleans up clang warning:
drivers/mmc/host/sh_mmcif.c:919:6: warning: Value stored to 'opc'
during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>