drivers: serial: nrf_uarte: Fix NO_OPTIMIZATION compilation

When NO_OPTIMIZATIONS is set and asynchronous API is used but
HW counting is not enabled then linking fails because of lack of
nrfx_timer code. When optimization is enabled, linker is smart
enough to figure out that nrfx_timer is not used.

Converting decision function from static inline function to macro
which is handled correctly with optimization off.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>