Bluetooth: AVDTP: Add AVDTP Discover Function Definition

Implementation of Send AVDTP_DISCOVER functionality

< ACL Data TX: Handle 256 flags 0x00 dlen 6
      Channel: 64 len 2 [PSM 25 mode 0] {chan 0}
      AVDTP: Discover (0x01) Command (0x00) type 0x00 label 0 nosp 0

Change-Id: Ieb6d1e5379d8933d57f3b1399f091378ce3c0756
Signed-off-by: Arun Jagadish <arun.jagadish@intel.com>