USB: gadget: dbgp: cleanup: remove unneeded check

len is always greater than or equal to zero here.  First of all, it's
type is unsigned and also we only assign it numbers which are greater
than or equal to zero.

Removing the check lets us pull everything in an indent level.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Stephane duverger <stephane.duverger@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>