lib: sbi: Counter info width should be zero indexed

The mhpm bits represent the number of bits available in mhpmcounter
while counter width describes a zero indexed value. Fix the counter width
calculation.

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