usb: gadget: mass_storage: remove >= 0 check for unsigned type

| In file included from drivers/usb/gadget/acm_ms.c:43:
| f_mass_storage.c:2199:18: warning: comparison of unsigned expression >= 0 is always true  [-Wtautological-compare]
|         if (common->lun >= 0 && common->lun < common->nluns)
|             ~~~~~~~~~~~ ^  ~

common->lun is defined as "unsigned int" so its value is always >= 0.
It is assigned via cbw->Lun which is defined as u8 so it is also not
abused as -1.

[ mina86@mina86.com : make lun unsigned int and use %u in DBG() macro for it ]

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>