memstick: r592: remove unused variable

Fix the following clang warning:

drivers/memstick/host/r592.c:363:6: warning: variable ‘len’ set but not
used [-Wunused-but-set-variable].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1618366903-94346-1-git-send-email-jiapeng.chong@linux.alibaba.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>