remove negative exit codes and other shellchecks in wedge_power.sh

[Scott8440]

Summary: I was getting shellcheck errors when editing this file in another diff. Negative exit codes and return values are not supported in bash: https://unix.stackexchange.com/questions/418784/what-is-the-min-and-max-values-of-exit-codes-in-linux

Test Plan: This is the only usage of it in code and it doesn't rely on a negative exit code (which couldn't happen anyways): https://www.internalfb.com/code/search?q=WEDGE_POWER_RESET_CMD

Differential Revision: D47138227

Reviewed By: smithscott

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This pull request was exported from Phabricator. Differential Revision: D47138227

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D47138227

[facebook-github-bot]

This pull request has been merged in facebook/openbmc@0554af8b462379b234c8219d1e9e4b2a5cceb1bb.