usb: gadget: f_mass_storage: Rework do_request_sense slightly

When building with clang, it notes that sdinfo may be unused
uninitialized in some cases. This appears to be true from reading the
code, and we can simply set the variable to zero to start with and be as
correct as before.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Marek Vasut <marex@denx.de>