util/lint/lint-stable-003-whitespace: Fix shell variable name

Fix shell variable "LINTDIR" so that helper_functions.sh can be found.

TEST=`./util/lint/lint lint-stable --junit` no longer prints "cannot
open /helper_functions.sh: No such file"

Signed-off-by: Fred Reitberger <reitbergerfred@gmail.com>
Change-Id: I68f2e65fa1c9297ad6b58b77576deaeef8bd76e3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68225
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

util/lint/lint-stable-003-whitespace

@@ -4,7 +4,7 @@
 
 # DESCR: Check for superfluous whitespace in the tree
 
-INTDIR="$(
+LINTDIR="$(
   cd -- "$(dirname "$0")" > /dev/null 2>&1 || return
   pwd -P
 )"