scripts: Address shellcheck issues

Report issues by shell files with:

    git ls-files '*.sh' | xargs shellcheck --exclude=SC2162

Address the following:

- SC1087: Use braces when expanding arrays
- SC1091: Not following
- SC2004: `$`/`${}` is unnecessary on arithmetic variables
- SC2024: `sudo` doesn't affect redirects
- SC2034: foo appears unused. Verify it or export it
- SC2086: Double quote to prevent globbing and word splitting
- SC2087: Quote `EOF`
- SC2115: Use `"${var:?}"` to ensure this never expands to `/*`
- SC2148: Add a shebang

Addresses (at least partially) some POSIX/dash issues:

- SC2113: `function` keyword is non-standard
- SC3010: In POSIX sh, `[[` `]]` is undefined
- SC3014: In POSIX sh, `==` in place of `=` is undefined
- SC3020: In POSIX sh, `&>` is undefined
- SC3046: In POSIX sh, `source` in place of `.` is undefined

Does not address:

- SC2162: `read` without `-r` will mangle backslashes
- Any other POSIX/dash-specific issues

Signed-off-by: Tim Crawford <tcrawford@system76.com>

scripts/realtek-coeffs.sh

@@ -13,7 +13,7 @@ do
 	codec_sys="/sys/class/sound/${codec_id}"
 	vendor="$(cat "${codec_sys}/vendor_name")"
 	chip="$(cat "${codec_sys}/chip_name")"
-	if [ "${vendor}" == "Realtek" ]
+	if [ "${vendor}" = "Realtek" ]
 	then
 		echo "# ${codec_id}: ${vendor} ${chip}"
 
@@ -27,7 +27,7 @@ do
 		do
 			# Set coefficient index
 			index_hex="$(printf "0x%02x\n" "${index}")"
-			hda-verb "${codec}" "${nid}" SET_COEF_INDEX "${index_hex}" &>/dev/null
+			hda-verb "${codec}" "${nid}" SET_COEF_INDEX "${index_hex}" >/dev/null 2>&1
 
 			# Get processing coefficient
 			value="$(hda-verb "${codec}" "${nid}" GET_PROC_COEF 0 2>/dev/null | cut -d " " -f 3)"