drivers: serial: Fix async to interrupt driven adaptation layer

Whenever UART_RX_DISABLED event is received module attempts to
re-enable receiver since in interrupt driven API receiver is
always on. However, it is possible that there is no free buffers
and in that case attempt to enable the receiver will fail with
-EBUSY. That scenario shall be accepted since the receiver will
be re-enabled when at least one buffer will be freed.

Given that, -EBUSY return shall be accepted (no assert) and
number of pending RX buffer requests shall be reset only on
successful enabling.

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