net/sixlowpan: Fix protosize to 16-bit

There was another small issue on sixlowpan_input.c code, it was
processing protosize and 8-bit instead of 16-bit.

It was working because the max tcp->tcpoffset was 0xf0, so
protosize = ((uint16_t)tcp->tcpoffset >> 4) << 2;
Will be protosize = 15 * 4 = 60 and will fit inside 8-bit.

Signed-off-of: Alan C. Assis <acassis@gmail.com>

Author: acassis

net/sixlowpan/sixlowpan_input.c

@@ -318,7 +318,7 @@ static int sixlowpan_frame_process(FAR struct radio_driver_s *radio,
   uint16_t paysize;           /* Size of the data payload */
   uint16_t fragtag   = 0;     /* Tag of the fragment */
   uint8_t fragoffset = 0;     /* Offset of the fragment in the IP packet */
-  uint8_t protosize  = 0;     /* Length of the protocol header (treated like payload) */
+  uint16_t protosize  = 0;    /* Length of the protocol header (treated like payload) */
   bool isfrag        = false; /* true: Frame is a fragment */
   bool isfrag1       = false; /* true: Frame is the first fragment of the series */
   int reqsize;                /* Required buffer size */