scripts: Fix shellcheck warnings for checkpatch scripts

- Switching from 'set -e' to 'set -euo pipefail' for better error handling.
- Quoting all variable expansions to prevent word splitting and globbing.
- Replacing legacy unquoted $(...) and $@ with quoted forms.
- Using array-safe idioms where necessary (e.g., "${src_files[@]}").

This patch doesn't change any functionality.

scripts/checkpatch_diff

@@ -1,13 +1,13 @@
 #!/usr/bin/env bash
 
-set -e
+set -euo pipefail
 
-rootdir=$(readlink -f $(dirname $0)/..)
-scriptsdir=${rootdir}/scripts
-base_commit=${1:-master}
+rootdir="$(readlink -f "$(dirname "$0")/..")"
+scriptsdir="${rootdir}/scripts"
+base_commit="${1:-master}"
 
 err=0
 
-(cd ${rootdir} && git diff ${base_commit} | ${scriptsdir}/checkpatch -) || err=1
+(cd "${rootdir}" && git diff "${base_commit}" | "${scriptsdir}/checkpatch" -) || err=1
 
-exit $err
+exit "${err}"