toolchain: kernel-headers: fix check target for kernel Git tree

Currently the check target fails if the kernel Git tree is used:

 $ make toolchain/kernel-headers/{download,check}

  make[2]: Entering directory 'toolchain/kernel-headers'
  Makefile:105: *** ERROR: Unknown pack format for file openwrt/tmp/dl/.  Stop.
  make[2]: Leaving directory 'toolchain/kernel-headers'
  toolchain/Makefile💯 recipe for target 'toolchain/kernel-headers/check' failed

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit bb7ba6b6a81d1fb7ac6075edfd8e8b713dd61db2)

toolchain/kernel-headers/Makefile

@@ -17,11 +17,18 @@ include $(INCLUDE_DIR)/kernel.mk
 PKG_NAME:=linux
 PKG_VERSION:=$(LINUX_VERSION)
 PKG_SOURCE:=$(LINUX_SOURCE)
-ifneq ($(strip $(CONFIG_KERNEL_GIT_CLONE_URI)),"")
+ifneq ($(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
   PKG_SOURCE_PROTO:=git
   PKG_SOURCE_URL:=$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI))
-  PKG_SOURCE_VERSION:=$(CONFIG_KERNEL_GIT_REF)
+  PKG_SOURCE_VERSION:=$(call qstrip,$(CONFIG_KERNEL_GIT_REF))
+ifdef CHECK
+  include $(INCLUDE_DIR)/kernel-version.mk
+  PKG_VERSION:=$(LINUX_VERSION)
 else
+  PKG_SOURCE:=$(LINUX_SOURCE)
+endif
+else
+  PKG_SOURCE:=$(LINUX_SOURCE)
   PKG_SOURCE_URL:=$(LINUX_SITE)
 endif
 HOST_BUILD_DIR:=$(KERNEL_BUILD_DIR)/linux-$(LINUX_VERSION)