serial_bt: silence compiler warning (unused value)

serial_bt: silence compiler warning (unused value)

Drop assignments of values to variables which never took effect. Drop
assignments which strictly speaking were not required, but were added in
the past to silence warnings in different compiler versions.

../src/serial_bt.c:168:2: warning: Value stored to 'type' is never read
        type = SER_BT_CONN_UNKNOWN;

../src/serial_bt.c:507:2: warning: Value stored to 'now_us' is never read
        now_us = 0;     /* Silence a (false) compiler warning. */

Shuffle variable declarations into the rough order in which they are
used in the function's body.