macintosh/macio-adb: check the return value of ioremap()

The function ioremap() in macio_init() can fail, so its return value
should be checked.

Fixes: 36874579

 ("[PATCH] powerpc: macio-adb build fix")
Reported-by: Hacash Robot <hacashRobot@santino.com>
Signed-off-by: Xie Shaowen <studentxswpy@163.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220802074148.3213659-1-studentxswpy@163.com