Move consts out of class level

[caternuson]

Fix for #12

The combination of the use of MicroPython's const() along with naming the variable with a leading _ was leading to incompatibility with non-native CP boards when used at the class level. This PR simply moves the consts to the module level.

Tested on Itsy:

Adafruit CircuitPython 5.3.1 on 2020-07-13; Adafruit ItsyBitsy M0 Express with samd21g18
>>> import board
>>> import busio
>>> import digitalio
>>> import adafruit_fram
>>> spi = busio.SPI(board.SCK, board.MOSI, board.MISO)
>>> cs = digitalio.DigitalInOut(board.D5)
>>> fram = adafruit_fram.FRAM_SPI(spi, cs)
>>> fram[0] = 1
>>> print(fram[0])
bytearray(b'\x01')
>>>

and RPI:

pi@raspberrypi:~/blinka/lib/python3.7/site-packages $ python3
Python 3.7.3 (default, Dec 20 2019, 18:57:59) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import board
>>> import busio
>>> import digitalio
>>> import adafruit_fram
>>> spi = busio.SPI(board.SCK, board.MOSI, board.MISO)
>>> cs = digitalio.DigitalInOut(board.D5)
>>> fram = adafruit_fram.FRAM_SPI(spi, cs)
>>> fram[0] = 1
>>> print(fram[0])
bytearray(b'\x01')
>>>

[ladyada]

thatnks, this was an 'odd one' to catch because i usually do it at file level :)

[caternuson]

yep. totes. this even resulted in some additions to the CP design guide docs.