bcma: fix handling of big addrl

The return value of bcma_erom_get_addr_desc() is a unsigned value and it
could wrap around in the two complement writing. This happens for one
core in the BCM4708 SoC.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>