Fix profile mode

[ReimuHakurei]

• Please check if the PR fulfills these requirements

• The changes have been tested locally
• There are no breaking changes

• What kind of change does this PR introduce?
  Fixes Profile mode: target temperature never settles #1939 -- this appears to most likely be a regression from baf2f26
  In addition to the bugs noted in that issue, I observed a couple of others that are also fixed here:

  • If two phases had the same target temperature, it would set the target to 0 degrees when hitting that phase.
  • The target temperature could underflow when cooling down.
  • The comment in the code indicated that it would reject entering profile mode if the actual temperature was greater than the defined preheat temperature, but the actual behavior was a hardcoded 55 C limit. I changed it to match what the comment said it should do.

• What is the current behavior?

• What is the new behavior (if this is a feature change)?

• Other information:
  I tested this same patch on the branch for the Sequre T55, but as this is the only supported hotplate I own, I can't test this on the dev branch, as the T55 support has not been merged into dev yet.

[grinningmosfet]

Successfully tested on an MHP30.