bitfield: suppress "dubious: x & !y" sparse warning

There's a somewhat common pattern of using FIELD_PREP()
even for single bits, e.g.

 cmd->info1 |= FIELD_PREP(HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP,
                          !!(params.flags & HAL_SRNG_FLAGS_MSI_SWAP));

which might as well be written as

 if (params.flags & HAL_SRNG_FLAGS_MSI_SWAP)
   cmd->info1 |= HTT_SRNG_SETUP_CMD_INFO1_RING_FLAGS_MSI_SWAP;

(since info1 is fully initialized to start with), but in
a long chain of FIELD_PREP() this really seems fine.

However, it triggers a sparse warning, in the check in
the macro for whether a constant value fits into the mask,
as this contains a "& (_val)". In this case, this really
is always intentional, so just suppress the warning by
adding "0+" to the expression, indicating explicitly that
this is correct.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240223100146.d243b6b1a9a1.I033828b1187c6bccf086e31400f7e933bb8373e7@changeid