endian: Wrap arguments in braces to silence warnings

When using the endian swapping marcos with multiple arguments that are
or'd together:

  CPU_TO_BE64(THIS_THING | THAT_THING)

gcc will emit this warning:

  warning: suggest parentheses around arithmetic in operand
  of ‘|’ [-Wparentheses]

Wrap the arugments in braces to avoid this.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>