lib: sbi: fix hart_shmem_base

When only phys_hi is equal to SBI_DBTR_SHMEM_INVALID_ADDR, it may be
a legal address. The old code would modify the legal address.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Himanshu Chauhan <hchauhan@ventanamicro.com>