[sserial] Resolved read issues

Changes:
  * Changed read rules to allow zero bytes read
  * Enabled async read and write - resource not available issue
  * Added timeout in 15sec on read/write
  * Added debug logs to read and write function on -1

Signed-off-by: Sergey Shcherbakov <shchers@gmail.com>
[sserial] fprintf replaced with logging module

Signed-off-by: Sergey Shcherbakov <shchers@gmail.com>
Static delay between AT commands replaced with waiting for result

Signed-off-by: Sergey Shcherbakov <shchers@gmail.com>