Don't read AUX_MON_TYPE if memory model is flat (#339)

Description
Add error handling to get_aux_mon_type API

Motivation and Context
get_aux_mon_type reads field consts.AUX_MON_TYPE in eeprom on page 1 address 145
If memory model is flat, there is no page 1 in eeprom, only 0_lower and 0_upper
Here get_aux_mon_type doesn't check whether memory model is not flat, this causes errors in running xcvrd

How Has This Been Tested?
Tested on testbed, assured that logs of failed eeprom reading disappeared

sonic_platform_base/sonic_xcvr/api/public/cmis.py

@@ -790,7 +790,7 @@ def get_aux_mon_type(self):
         '''
         This function returns the aux monitor types
         '''
-        result = self.xcvr_eeprom.read(consts.AUX_MON_TYPE)
+        result = self.xcvr_eeprom.read(consts.AUX_MON_TYPE) if not self.is_flat_memory() else None
         if result is None:
             return None
         aux1_mon_type = result & 0x1