lm-sensors: fix missing PKG_SOURCE variable (#2933)

Fix missing `PKG_SOURCE` variable leading to a
`Download/default is missing the FILE field` error in the buildroot.

Also remove now uneeded  variable as the default one matches the git clone
structure. Finally use the common variable assignment style for
`PKG_SOURCE_PROTO` and `PKG_SOURCE_VERSION`.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

utils/lm-sensors/Makefile

@@ -11,14 +11,14 @@ PKG_NAME:=lm-sensors
 PKG_VERSION:=3.3.5
 PKG_RELEASE:=3
 
-PKG_SOURCE_PROTO=git
+PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/groeck/lm-sensors.git
-PKG_SOURCE_VERSION=f8cdcc35bff0785aecf49d9a8484a71ce3ebee4f
+PKG_SOURCE_VERSION:=f8cdcc35bff0785aecf49d9a8484a71ce3ebee4f
 PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
+PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_MAINTAINER:=Jo-Philipp Wich <jo@mein.io>
 PKG_LICENSE:=GPL-2.0+ LGPL-2.1+
 
-PKG_BUILD_DIR:=$(BUILD_DIR)/lm_sensors-$(PKG_VERSION)
 PKG_BUILD_PARALLEL:=1
 
 include $(INCLUDE_DIR)/package.mk