eal: explicit cast in constant byte swap

GCC 8.1 warns:

rte_byteorder.h: In function 'rte_constant_bswap16':
rte_byteorder.h:54:45: warning: conversion from
'int' to 'uint16_t' {aka 'short unsigned int'}
may change value [-Wconversion]
  ((((uint16_t)(v) & UINT16_C(0x00ff)) << 8) | \
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
   (((uint16_t)(v) & UINT16_C(0xff00)) >> 8))
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
rte_byteorder.h:126:9: note: in expansion of macro
'RTE_STATIC_BSWAP16'
  return RTE_STATIC_BSWAP16(x);
         ^~~~~~~~~~~~~~~~~~

The other two sizes are going to be afflicted the
same, so get the same fix.

Fixes: b75667ef9f7e ("eal: add static endianness conversion macros")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>