eeprom: add support to use CBUS IntEnum in _set_cbus_func

[talhaHavadar]

This commit adds the capability of using FtdiEeprom.CBUS, CBUSH and CBUSX while setting cbus functionality as shown in the example below.

	eeprom = FtdiEeprom()
	eeprom.connect(f) # f is the instance of Ftdi
	eeprom.set_property("cbus_func_0", eeprom.CBUSX.GPIO)

Previously, we had to use the string to set the property for specific CBUS. And when we want to use it like the example above it was giving the following error messages:

Traceback (most recent call last):
  File "/home/talha/projects/pyftdi/test.py", line 45, in <module>
    eeprom.set_property("cbus_func_0", eeprom.CBUSX.GPIO)
  File "/home/talha/projects/pyftdi/pyftdi/eeprom.py", line 490, in set_property
    raise ValueError("'{name}' should be specified as a string")
ValueError: '{name}' should be specified as a string

Traceback (most recent call last):
  File "/home/talha/projects/pyftdi/test.py", line 45, in <module>
    eeprom.set_property("cbus_func_0", eeprom.CBUSX.GPIO)
  File "/home/talha/projects/pyftdi/pyftdi/eeprom.py", line 491, in set_property
    self._set_cbus_func(int(mobj.group(1)), value, out)
  File "/home/talha/projects/pyftdi/pyftdi/eeprom.py", line 896, in _set_cbus_func
    code = cbus[value.upper()].value
AttributeError: 'CBusX' object has no attribute 'upper'