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 <mike.looijmans@topic.nl>
Closes: https://lore.kernel.org/all/efae4037-c22a-40be-8ba9-7c1c12ece042@topic.nl/


Fixes: 4a679c5a ("selftests: Add test to verify power supply properties")
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>