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

drivers/usb/gadget/legacy/mass_storage.c: In function msg_do_config:
drivers/usb/gadget/legacy/mass_storage.c:108:19: warning: variable opts set but not used [-Wunused-but-set-variable]

It is not used since commit f78bbcae ("usb: f_mass_storage:
test whether thread is running before starting another")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>