[AVR] Replace two references to ARM's 't2_so_imm' type comments

These were originally introduced in a copy-paste committed in r351526.

The reference to 't2_so_imm' have been updated to 'imm_com8' so the
comment is now accurate.

Thanks to Eli Friedman for noticing this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351674 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed