sleep: fix flushing UARTs when entering deep sleep

sleep: fix flushing UARTs when entering deep sleep

Since 94250e42a0, UART output is suspended when entering sleep mode
(deep or light sleep). This makes sense for light sleep, where sleep
normally takes small amount of time, and flushing the UART would add
a lot of latency. But this breaks existing behaviour for deep sleep,
where UART output was previously sent out before entering sleep mode.

Closes https://github.com/espressif/esp-idf/issues/2145