lib: fix csr detect support

csr_read_allowed/csr_read_allowed requires trap.case to detect the results,
but if no exception occurs, the value of trap.case will remain unchanged,
which makes the detection results unreliable. Add code to initialize
trap.case to 0.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>