usb: gadget: f_mass_storage: style corrections, cleanup & simplification

Fix spacing, improve error code returned, remove unused #define,
use strtobool() instead of kstrtou8().

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>