Update rom.c

Shadowrom on - do nothing if shadowrom is already active. ANDing r1 and r2 with 0xF0 to avoid junk on bits 3-0.
Merge pull request #3 from rafgc/patch-3

Shadowrom on - do nothing if shadowrom is already active. ANDing r1 and r2 with 0xF0 to avoid junk on bits 3-0.