drivers: serial: nrfx_uarte: Minor code size optimization

Optimize function which enables UARTE peripheral. It is called
only when interrupts are locked so data->flags does not require
atomic operation. Use standard logical operations so save few
bytes.

Simplify uarte_disable_locked.

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