Fixed redefinition warning

[remenyo]

Merged the ADC_BITS ifdef statement, used an #elif defined(ESP32) instead of another #if defined(ESP32) statement
This way the compiler will not complain about redeclaring ADC_BITS.

The warning was this:

.pio\libdeps\nodemcu-32s\EmonLib-esp32/EmonLib.h:43:0: warning: "ADC_BITS" redefined
 #define ADC_BITS    12
 ^
.pio\libdeps\nodemcu-32s\EmonLib-esp32/EmonLib.h:39:0: note: this is the location of the previous definition
 #define ADC_BITS    10
 ^