Optimize receiver sensitivity, per errata note 2.1

[elena-v2]

According to Errata Note § 2.1 for the SX1276/77/78/79 series

The following LoRa registers should be changed as described, for BW=500 kHz

• For carrier frequencies ranging from 862 to 1020 MHz
  • Set LoRa register at address 0x36 to value 0x02 (by default 0x03)
  • Set LoRa register at address 0x3a to value 0x64 (by default 0x65)
• For carrier frequencies ranging from 410 to 525 MHz
  • Set LoRa register at address 0x36 to value 0x02 (by default 0x03)
  • Set LoRa register at address 0x3a to value 0x7F (by default 0x65)

For all other combinations of bandwidth / frequencies, register at address 0x36 should be re-set to value 0x03, and the value at address 0x3a will be automatically selected by the chip.

This commit does exactly that.

[markqvist]

Thank you very much! Will be included in next release :)