usb: gadget: Remove the LUN checks which are always true

Comparing an unsigned integer with greater than or equal to zero is
always true.  So, it is safe to remove similar checks from
'f_mass_storage.c' and 'file_storage.c'

Signed-off-by: Maxin B. John <maxin.john@gmail.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>