firmware: 2modpow_sse2: Clean up calculation of `mu`

The expression `(uint32_t)(1 << 32)` is just zero, so we can simplify
the calculation of `mu` to `-key->n0inv`.

BRANCH=none
BUG=none
TEST=none

Change-Id: I20bec985ac4ebdabe2f626ce3273d6dd2bc69a1e
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/5224973
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Jeremy Compostella <jeremy.compostella@intel.com>