Fix NumRegField byte(s) read (#392)

Description
NumRegField read worked fine when the fields were single bit. Now that we have RegBitsFields with multiple bits we need to adjust the mask accordingly.

Motivation and Context
NumRegFields having RegBitsFields did not read the byte correctly. This is now fixed with this change.

sonic_platform_base/sonic_xcvr/fields/xcvr_field.py

@@ -20,6 +20,7 @@ def __init__(self, name, offset, **kwargs):
         self.offset = offset
         self.ro = kwargs.get("ro", True)
         self.deps = kwargs.get("deps", [])
+        self.bitmask = None
 
     def get_fields(self):
         """
@@ -86,6 +87,7 @@ def __init__(self, name, bitpos, offset=None, **kwargs):
         super(RegBitField, self).__init__(name, offset, **kwargs)
         assert bitpos < 64
         self.bitpos = bitpos
+        self.bitmask = 1 << self.bitpos
 
     def get_size(self):
         return 1
@@ -115,7 +117,7 @@ class RegBitsField(XcvrField):
     def __init__(self, name, bitpos, offset=None, **kwargs):
         super(RegBitsField, self).__init__(name, offset, **kwargs)
         self.size = self.size = kwargs.get("size", 1) #No of bits
-        assert (bitpos >= 0 and (bitpos+self.size-1 < 8), "bitpos must be within one byte")
+        assert bitpos >= 0 and bitpos+self.size <= 8, "bitpos must be within one byte"
         self.bitpos = bitpos
         self.bitmask = (((1 << self.size) - 1) << self.bitpos) & 0xff
 
@@ -159,7 +161,7 @@ def get_bitmask(self):
             return None
         mask = 0
         for field in self.fields:
-            mask |= 1 << field.bitpos
+            mask |= field.bitmask
         return mask
 
     def get_size(self):