usb: dwc3: debugfs: fix off by one when entering testmode

When implementing the USB2 testmode support via debugfs,
Felipe has committed a mistake when counting the number
of letters of some of the strings, resulting on an off
by one error which prevented some of the Test modes to
be entered properly.

This patch, fixes that mistake.

Signed-off-by: Gerard Cauvy <g-cauvy1@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>