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

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

drivers/memstick/host/tifm_ms.c: In function 'tifm_ms_issue_cmd':
drivers/memstick/host/tifm_ms.c:259:17: warning:
 variable 'data' set but not used [-Wunused-but-set-variable]

It's not used any more since commit 92b22d93 ("tifm: fix the
MemoryStick host fifo handling code")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>