fix compiler warning - convertSpektrumVtxPowerIndex - unused parameter 'sPower'

[nerdCopter]

• fixes:

./src/main/io/spektrum_vtx_control.c: In function 'convertSpektrumVtxPowerIndex':
./src/main/io/spektrum_vtx_control.c:115:46: warning: unused parameter 'sPower' [-Wunused-parameter]
  115 | uint8_t convertSpektrumVtxPowerIndex(uint8_t sPower) {

• only Targets affected were Beesign targets, therefore safe to merge.
  • HMBF4PRO_US
  • NBDHMBF4PRO (tested; analog output still working)
  • HMBF4PRO_INTL