lib: sbi: Resolve the uninitialized complaint in sbi_pmu

A recent commit (b28f0700) turns to read from CSR_MCOUNTINHIBIT after
checking the SBI_HART_HAS_MCOUNTINHIBIT HART feature, which leaves the
mctr_inhbt uninitialized in pmu_ctr_find_hw().
As a result, compiler will complain:
error: 'mctr_inhbt' may be used uninitialized in this function.

This commit resolves the issue by assigning an initial value to mctr_inhbt.

Signed-off-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>