usb: gadget: f_mass_storage: simplify start_transfer slightly

Flatten the start_transfer function by reversing the if condition and
returning early out of the function if everything went fine.  It makes
the function look less complicated, at least to me, and easier to
understand.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>