rex3: frame narrow DCB stores by access width, not store address

[kaloz]

write8 and write16 position a narrow store inside the 32-bit dcb_write value by the store's low address bits, but REX3_DCBDATA0 is a fixed auto-incrementing port: the hardware left-justifies a narrow value by its access width. A halfword store at the port's base address gets shift 0 instead of 16, dcb_write's MSB-first extraction reads two zero bytes, and every CMAP ADDRESS_LOW write lands on palette index 0 - NetBSD's console palette collapses into slot 0 and the console renders white on white. Shift by the access width instead.