Power optimization - Improve SPI sleep mode

Ensure that all pins are set to their default configuration during sleep mode.
Disable the workaround for FTPAN58 (SPI freezes when transfering a single byte) at the end of the transfer. This disables the resources needed for the workaround.
Those changes reduce the power usage by 430-490µA.

src/drivers/Spi.cpp

@@ -27,7 +27,8 @@ bool Spi::WriteCmdAndBuffer(const uint8_t* cmd, size_t cmdSize, const uint8_t* d
 }
 
 bool Spi::Init() {
-  nrf_gpio_pin_set(pinCsn); /* disable Set slave select (inactive high) */
+  nrf_gpio_cfg_output(pinCsn);
+  nrf_gpio_pin_set(pinCsn);
   return true;
 }