drivers: serial: nrfx_uarte: Optimize code size

Code size can significantly reduced (220 bytes per instance) if data
structure (stored in RAM) is not initilized (can be moved to .bss).
Data for asynchronous API had two fields which can easily be moved
to the configuration structure (which is in ROM) because they do not
change duing runtime.

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