drivers/onewire: Fix undefined variable errors.

On CPython, and with pylint, the variables MATCH_ROM and SEARCH_ROM are
undefined.  This code works in MicroPython because these variables are
constants and the MicroPython parser/compiler optimises them out.  But it
is not valid Python because they are technically undefined within the scope
they are used.

This commit makes the code valid Python code.  The const part is removed
completely because these constants are part of the public API and so cannot
be moved to the global scope (where they could still use the MicroPython
const optimisation).

drivers/onewire/onewire.py

 # 1-Wire driver for MicroPython
 # MIT license; Copyright (c) 2016 Damien P. George
 
-from micropython import const
 import _onewire as _ow
 
 
@@ -10,9 +9,9 @@ class OneWireError(Exception):
 
 
 class OneWire:
-    SEARCH_ROM = const(0xF0)
-    MATCH_ROM = const(0x55)
-    SKIP_ROM = const(0xCC)
+    SEARCH_ROM = 0xF0
+    MATCH_ROM = 0x55
+    SKIP_ROM = 0xCC
 
     def __init__(self, pin):
         self.pin = pin
@@ -46,7 +45,7 @@ class OneWire:
 
     def select_rom(self, rom):
         self.reset()
-        self.writebyte(MATCH_ROM)
+        self.writebyte(self.MATCH_ROM)
         self.write(rom)
 
     def scan(self):
@@ -64,7 +63,7 @@ class OneWire:
     def _search_rom(self, l_rom, diff):
         if not self.reset():
             return None, 0
-        self.writebyte(SEARCH_ROM)
+        self.writebyte(self.SEARCH_ROM)
         if not l_rom:
             l_rom = bytearray(8)
         rom = bytearray(8)