bcma: Drop the unused parameter of bcma_scan_read32()

As it had never been used since the initial commit 8369ae33 ("bcma: add
Broadcom specific AMBA bus driver").

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210727025232.663-3-yuzenghui@huawei.com