shell: uart: Rework Kconfig dependencies

When UART asynchronous API support was added to shell it was set up
to be the default one and was turning on asynchronous API if it was
supported. However, it may lead to complation failures if device
requires additional setup for asynchronous UART (e.g DMA in device
tree). Becuase of that, it is reverted back to use interrupt driven
API and use asynchronous API if it is already enabled in the
application.

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