usb: gadget: dummy-hcd: Remove utf8 from format string

Not everybody uses a utf8 locale (unfortunately), so let's avoid
non-ascii characters in the kernel log. Replace the 3-byte utf8
sequence with a 3-byte ascii equivalent.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>