db845c: Switch default TARGET_KERNEL_USE to 5.4

It was requested that the default kernel be 5.4
based for db845c builds, so lets switch this.

Change-Id: I4ba0ce9916e63457a114d51cbe28ec7f830a1e2c
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/db845c.mk b/db845c.mk
index 721a9ad..a4984f0 100644
--- a/db845c.mk
+++ b/db845c.mk
@@ -1,5 +1,5 @@
 ifndef TARGET_KERNEL_USE
-TARGET_KERNEL_USE := mainline
+TARGET_KERNEL_USE := 5.4
 endif
 DB845C_KERNEL_DIR := device/linaro/dragonboard-kernel/android-$(TARGET_KERNEL_USE)