m68k: Fix bug, "address of" operator was forgotten

in_be16() shall be passed a pointer to register and not its value. This
is clearly a typo resulting in a wrong memory access, so fix it.

Cc: Alison Wang <b18965@freescale.com>, Jason Jin <Jason.jin@freescale.com>
Signed-off-by: Vasili Galka <vvv444@gmail.com>