lib: sbi: simplify pmp_set(), pmp_get()

pmpcfg_csr and pmpcfg_shift are only negative for an unexpected value of
__riscv_xlen. We can immediately return in this case.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Anup Patel <anup.patel@wdc.com>