math/bits: remove named return in TrailingZeros16

math/bits: remove named return in TrailingZeros16

TrailingZeros16 is the only one of the TrailingZeros functions with a
named return value in the signature. This creates a sligthly
unpleasant effect in the godoc listing:

  func TrailingZeros(x uint) int
  func TrailingZeros16(x uint16) (n int)
  func TrailingZeros32(x uint32) int
  func TrailingZeros64(x uint64) int
  func TrailingZeros8(x uint8) int

Since the named return value is not even used, remove it.

Change-Id: I15c5aedb6157003911b6e0685c357ce56e466c0e
Reviewed-on: https://go-review.googlesource.com/c/153340
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>