uefi: Consistently use &mut self for pxe::BaseCode methods

[nicholasbishop]

Although these methods are just reading data, they are almost certainly modifying internal state in the UEFI protocol and so should be &mut.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

good catch