target/riscv: Fix up masking of vsip/vsie accesses

target/riscv: Fix up masking of vsip/vsie accesses

The current logic attempts to shift the VS-level bits into their correct
position in mip while leaving the remaining bits in-tact. This is both
pointless and likely incorrect since one would expect that any new, future
VS-level interrupts will get their own position in mip rather than sharing
with their (H)S-level equivalent. Fix this, and make the logic more
readable, by just making off the VS-level bits and shifting them into
position.

This also fixes reads of vsip, which would only ever report vsip.VSSIP
since the non-writable bits got masked off as well.

Fixes: d028ac7512f1 ("arget/riscv: Implement AIA CSRs for 64 local interrupts on RV32")
Signed-off-by: Andrew Bresticker <abrestic@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221215224541.1423431-1-abrestic@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>