DSP: fix compiler warning on inline attribute

When trying to compile the distance test on zephyr,
The compiler reports: "warning: 'inline' attribute
directive ignored". To fix this, we use `static inline`
explicitly instead on using the `attribute` flag.

In addition, to force inline we use the attribute
`always_inline`.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>