Fixed memory leak when using transport:serial.

Fixed memory leak when using transport:serial. Code was previously
using strdup() to make a copy of the serial number which was not
necessary. Instead, just set serial=service.

Change-Id: Ifa8fe7999c6fb88955d2c6bdfcd3cd724ec1f020
Signed-off-by: Tom Marlin <tomm@bsquare.com>