target/riscv/cpu.c: use offset in isa_ext_is_enabled/update_enabled

target/riscv/cpu.c: use offset in isa_ext_is_enabled/update_enabled

We'll have future usage for a function where, given an offset of the
struct RISCVCPUConfig, the flag is updated to a certain val.

Change all existing callers to use edata->ext_enable_offset instead of
'edata'.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230912132423.268494-14-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>