usb: gadget: Remove set but not used variable 'opts' in msg_do_config

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>
Link: https://lore.kernel.org/r/1570610434-68293-3-git-send-email-zhengbin13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>