scsi: hisi_sas: Make slot buf minimum allocation of PAGE_SIZE

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

For a system with PAGE_SIZE of 16K or 64K, the size every time we want to
alloc may be small like 4K, but for function dmam_alloc_coherent(), the
least size it allocates is PAGE_SIZE, so it will waste much memory for the
situation.

To solve the issue, limit the minimum allocation size of slot buf to
PAGE_SIZE.

Signed-off-by: Wang Chao <wangchao342@hisilicon.com>
Reviewed-by: Zhu Xiongxiong <zhuxiongxiong@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>