ipq40xx: fix 4.14 ImageBuilder build woes

ipq40xx: fix 4.14 ImageBuilder build woes

Robert Marko reported an issue with the current imagebuilder images:
"Imagebuilder includes the new kmod-usb-dwc3-qcom USB driver
package by default even on 4.14. [...] the current state imagebuilder
can't build images under 4.14 at all as the kmod-usb-dwc3-qcom does
not exist in it so it throws and error and exits."

This patch reverts the Makefile to just kmod-usb-dwc3-of-simple and
once the switch to 4.19 is done. It also removes the
kmod-usb-phy-qcom-dwc3 as they only contain the usb-phy drivers for
the ipq806x generation.

Dynamic switching based on the KERNEL_PATCHVER is possible by using:
   $(if $(filter 4.14,$(KERNEL_PATCHVER)),kmod-usb-dwc3-of-simple,kmod-usb-dwc3-qcom)
though it

Fixes: 13321fa1429 ("ipq40xx: Use kmod-usb-dwc3-qcom by default")
Fixes: 6e58fb2c33e ("ipq40xx: kmod-usb-dwc3-of-simple vs kmod-usb-dwc3-qcom")
Reported-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>