fix(bhi, leds): fixed faulty error handling due to always-false unsigned comparisons

-Wtype-limits revealed three errors in the firmware:

../pycardium/modules/sys_leds.c: In function 'mp_leds_get_rocket':
../pycardium/modules/sys_leds.c:207:10: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  207 |  if (ret == -EINVAL) {
      |          ^~


../pycardium/mphalport.c: In function 'mp_hal_set_interrupt_char':
../pycardium/mphalport.c:115:8: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  115 |  if (c != -1) {
      |        ^~

../epicardium/modules/bhi.c: In function 'epic_bhi160_enable_sensor':
../epicardium/modules/bhi.c:134:12: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  134 |  if (vs_id < 0) {
      |            ^
../epicardium/modules/bhi.c: In function 'epic_bhi160_disable_sensor':
../epicardium/modules/bhi.c:191:12: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  191 |  if (vs_id < 0) {
      |            ^

note, that on the target platform char and enums are unsigned. The compiler has probably removed all of these ifs so far because the conditions were determined to always be false.