bcma: add missing iounmap on error path

This should fix the problem reported by Fengguang:

The coccinelle static checker emits these warnings:

drivers/bcma/scan.c:466:3-9: ERROR: missing iounmap; ioremap on line 451 and execution via conditional on line 465
drivers/bcma/scan.c:540:3-9: ERROR: missing iounmap; ioremap on line 515 and execution via conditional on line 539

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>