AP_HAL_Linux: Correcting soc filename size in file detection.

[massout]

soc filename size for comparison has changed from 4 to 3. Without this change, you might be getting a "ranges" file not found error.

[tpwrules]

What does ls /proc/device-tree/ look like on an affected device?

[massout]

$ ls /proc/device-tree/
'#address-cells'   aliases    cam0_reg        chosen     compatible    firmwarekms      interrupt-parent   model   pwr_button        sd-io-1v8-reg    system
'#size-cells'      arm-pmu    cam1_clk        clk-108M   cooling_fan   hvs@107c580000   leds               name    reserved-memory   sd-vcc-reg       thermal-zones
 __overrides__     axi        cam1_reg        clk-27M    cpus          i2c0if           memory@0           phy     rp1_firmware      serial-number    timer
 __symbols__       cam0_clk   cam_dummy_reg   clocks     dummy         i2c0mux          memreserve         psci    rp1_vdd_3v3       soc@107c000000   wl-on-reg