lps33thw: add syscfg to make statistics optional

[benmccrea]

Adds a syscfg, LPS33THW_STATS, to make use of statistics in the driver optional. The syscfg defaults to 1 (no modification to existing behavior). Some people will not care to use statistics for memory reasons.

I also want to note, the way statistics are currently implemented in the driver prevents it from being used with more than 1 sensor at a time, because lps33thw_init will attempt to register the statistics again the second time it is called. This will result in a sysinit panic:

    /* Initialise the stats entry */
    rc = stats_init(STATS_HDR(g_lps33thwstats),
        STATS_SIZE_INIT_PARMS(g_lps33thwstats, STATS_SIZE_32),
        STATS_NAME_INIT_PARMS(lps33thw_stat_section));
    SYSINIT_PANIC_ASSERT(rc == 0);

    /* Register the entry with the stats registry */
    rc = stats_register(dev->od_name, STATS_HDR(g_lps33thwstats));
    SYSINIT_PANIC_ASSERT(rc == 0);

It would be preferable to change this code so that each sensor has its own statistics, or if all sensors are to share the same statistics, then only call stats_init and stats_register for the first sensor.

[wes3]

LGTM