drivers: esp_at: make esp_pull_quoted() to only return -EAGAIN

If no " character is found in buffer by esp_pull_quoted() it
returns -EAGAIN which causes a loop that never ends. This is because
the buffer dont get filled up with new data so no " chrachter will be
found. This commit changes esp_pull_quoted() to only return -EAGAIN,
so the buffer can get filled with new data and thus the loop can come
to an end.

Signed-off-by: John Johnson <john.filip.johnson@gmail.com>