memstick: jmb38x_ms: remove set but not used variable 'data'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/memstick/host/jmb38x_ms.c: In function 'jmb38x_ms_issue_cmd':
drivers/memstick/host/jmb38x_ms.c:371:17: warning:
 variable 'data' set but not used [-Wunused-but-set-variable]

It's never used since introduction and can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>