bcma: Fix mem leak in bcma_bus_scan()

bcma_bus_scan() leaks 'struct bcma_device' bytes if
bcma_get_next_core() returns error.

Restructure the code so we always kfree() the memory we allocate to
the variable 'core' before it goes out of scope.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>