hotplug-drm: fixes shellcheck warnings
hotplug-drm: fixes shellcheck warnings

Reported by:

	$ make check
	shellcheck hotplug-drm hotplug-monitor

	In hotplug-drm line 38:
		if [ "$ACTION" = "remove" ]; then
	              ^-- SC2153: Possible misspelling: ACTION may not be assigned, but action is.

	In hotplug-drm line 40:
			rm -rf "$tmpdir/$name"
	                       ^-- SC2115: Use "${var:?}" to ensure this never expands to / .

	make: *** [Makefile:207: check] Error 1

gazoo74/udev-rules,gazoo74/udev-rules