tests: lib: cmsis_dsp: Fix -Wsometimes-uninitialized warning

Building with clang warns:

tests/lib/cmsis_dsp/distance/src/u32.c:82:3: error: variable 'val' is
used uninitialized whenever switch default is taken
[-Werror,-Wsometimes-uninitialized]
                default:
                ^~~~~~~
tests/lib/cmsis_dsp/distance/src/u32.c:87:19: note: uninitialized use
occurs here
                output[index] = val;
                                ^~~
tests/lib/cmsis_dsp/distance/src/u32.c:47:16: note: initialize the
variable 'val' to silence this warning
                float32_t val;
                             ^
                              = 0.0

Hitting the default switch will assert, so this won't cause any problems
as written, but it doesn't hurt to initialize the variable.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>