hw/fmopl: Fix buffer access out-of-bounds errors

Index 75 is one too large for AR_TABLE[75], DR_TABLE[75].
This error was reported by cppcheck.

hw/fmopl.c:600: error: Buffer access out-of-bounds: OPL.AR_TABLE
hw/fmopl.c:601: error: Buffer access out-of-bounds: OPL.DR_TABLE

Fix this by limiting the access to the allowed range.
MultiArcadeMachineEmulator has newer versions of fmopl,
but using these requires more efforts.

Cc: Blue Swirl <blauwirbel@gmail.com>
Reviewed-by: malc <av1474@comtv.ru>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
