lib: sbi: Fix initial value mask while updating the counters

The first 32 bits of the initial value for the counter should be
preserved while updating the mhpmcounter for 32bit.

Fixes: 13d40f21 ("lib: sbi: Add PMU support")
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Anup Patel <anup.patel@wdc.com>