serial: lower severity of failed stream detect message

serial: lower severity of failed stream detect message

Reduce the message's log level from ERR to INFO when the serial stream
detect routine cannot find a valid packet. Although an error code gets
returned, the condition need not be fatal (can be the result of trying
several peers before success or finally giving up). Let callers decide
on the severity of failure to detect a specific device's presence, and
provide more context in their message which is more helpful to users.
Interested readers still get the message at slightly raised log levels.

This avoids confusing user perceivable situations like these:

  $ sigrok-cli --scan
  sr: serial: Didn't find a valid packet (read 0 bytes).
  sr: serial: Didn't find a valid packet (read 0 bytes).
  The following devices were found:
  demo - Demo device with 13 channels: D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3 A4