mpu6000: use a wrapper struct to avoid a linker error

the linker doesn't cope with us having multiple modules implementing
RingBuffer<gyro_report>

this also switches to use force() instead of put(), so we discard old
entries when the buffer overflows
mpu6000: fixed race condition in buffer increment
airspeed: convert to using RingBuffer class
l3gd20: convert to using RingBuffer class
mb12xx: convert to using RingBuffer class
bma180: convert to using RingBuffer
lsm303d: convert to using RingBuffer
ms5611: converted to using RingBuffer