usb: dwc3: debugfs: improve debugfs file creation

when commit 388e5c51 (usb: dwc3: remove dwc3
dependency on host AND gadget.) changed the
way debugfs files are created, it failed to
note that 'mode' is necessary in Dual Role
mode only while 'testmode' and 'link_state'
are valid in Dual Role and Peripheral-only
builds. Fix this while also converting pre-
processor conditional to C conditionals.

Signed-off-by: Felipe Balbi <balbi@ti.com>