Update the Helium SubBand for US 915

[dhineshkumarmcci]

Change the SubBand of Helium from SubBand_7 to SubBand_2.

arduino-lorawan/src/lib/helium_us915_netbeginregioninit.cpp

Line 49 in a0577e1

cLMIC::SelectSubBand(cLMIC::SubBand::SubBand_7);