bitfield: Add less-checking __FIELD_{GET,PREP}()

The BUILD_BUG_ON_MSG() check against "~0ull" works only with "unsigned
(long) long" _mask types.  For constant masks, that condition is usually
met, as GENMASK() yields an UL value.  The few places where the
constant mask is stored in an intermediate variable were fixed by
changing the variable type to u64 (see e.g. [1] and [2]).

However, for non-constant masks, smaller unsigned types should be valid,
too, but currently lead to "result of comparison of constant
18446744073709551615 with expression of type ... is always
false"-warnings with clang and W=1.

Hence refactor the __BF_FIELD_CHECK() helper, and factor out
__FIELD_{GET,PREP}().  The later lack the single problematic check, but
are otherwise identical to FIELD_{GET,PREP}(), and are intended to be
used in the fully non-const variants later.

[1] commit 5c667d5a5a3ec166 ("clk: sp7021: Adjust width of _m in
    HWM_FIELD_PREP()")
[2] commit cfd6fb45cfaf46fa ("crypto: ccree - avoid out-of-range
    warnings from clang")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://git.kernel.org/torvalds/c/5c667d5a5a3ec166 [1]
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

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