* Extend mapper 28 (Action 53)'s PRG range from 2mbit to 8mbit
"Register" $81 (outer bank, i.e. regs[3]) supports a full 8-bit value
for the outer PRG ROM bank, permitting up to 8mbit total space.
Currently Mesen only honours the lower 6 bits (probably because some
(but not all) hardware implements it that way). This limit lead to some
complications when trying to use Mesen to test behaviour vs. actual
hardware.
Matter was discussed on Discord with jroach and Tepples.
* Fixup for 0f9d8a2: missed one line
* Fixup for 0f9d8a2: remove WriteRegister mask
