Replace or remove invalid characters in comments

Fixes flawfinder-sast failures:
Error: encoding error in ./src/xg47_videohw.c
'utf-8' codec can't decode byte 0x92 in position 1980: invalid start byte
Error: encoding error in ./src/xg47_regs.h
'utf-8' codec can't decode byte 0xa8 in position 6460: invalid start byte
Error: encoding error in ./src/xg47_regs.h
'utf-8' codec can't decode byte 0xe7 in position 4716: invalid continuation byte

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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