[LLDB][MIPS] Fix TestExprsChar.py

Patch by Nitesh Jain.

Summary: When incorrect type used for 'char' then (at least) one of the expression evaluates to incorrect value. Please refer to bug llvm.org/pr23069

Reviewers: ovyalov, clayborg
Subscribers: lldb-commits, mohit.bhakkad, sagar, bhushan, jaydeep
Differential: reviews.llvm.org/D16132
llvm-svn: 258684