serial_recovery: Reduce minimum number of members in packet

Was 3, but mcumgr sometimes sends 2 ("off" + "data", "len" is not needed
when "off" != 0). Reduce to 1 to avoid the problem, since the code
has other checks to catch this.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>