drivers: serial: nrfx_uarte: Refactor to use RX async struct pointer

Refactor RX asynchronous API function to use a pointer to the RX
async data structure instead of top level data structure pointer.
It improves readability with more concise code.

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