selftests: power_supply: Make it POSIX-compliant

There is one use of bash specific syntax in the script. Change it to the
equivalent POSIX syntax. This doesn't change functionality and allows
the test to be run on shells other than bash.

Reported-by: Mike Looijmans <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Fixes: 4a679c5 ("selftests: Add test to verify power supply properties")
Signed-off-by: Nícolas F. R. A. Prado <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

Author: nfraprado

tools/testing/selftests/power_supply/test_power_supply_properties.sh

@@ -23,7 +23,7 @@ count_tests() {
 	total_tests=0
 
 	for i in $SUPPLIES; do
-		total_tests=$(("$total_tests" + "$NUM_TESTS"))
+		total_tests=$((total_tests + NUM_TESTS))
 	done
 
 	echo "$total_tests"