drivers: sensor: max31855: fixed sign bit positions

[araneusdiadema]

MAX31855 driver had a wrong position of sign bits causing invalid reading of negative temperature values. Fixed by shifting position of sign bit by one bit.

[github-actions]

Hello @araneusdiadema, and thank you very much for your first pull request to the Zephyr project!
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[github-actions]

Hi @araneusdiadema!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

To celebrate this milestone and showcase your contribution, we'd love to award you the Zephyr Technical Contributor badge. If you're interested, please claim your badge by filling out this form: Claim Your Zephyr Badge.

Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁