hwrng: core - avoid kernel-doc warnings

hwrng: core - avoid kernel-doc warnings

Mark internal fields as "private:" so that kernel-doc comments
are not needed for them, eliminating kernel-doc warnings:

Warning: include/linux/hw_random.h:54 struct member 'list' not described
 in 'hwrng'
Warning: include/linux/hw_random.h:54 struct member 'ref' not described
 in 'hwrng'
Warning: include/linux/hw_random.h:54 struct member 'cleanup_work' not
 described in 'hwrng'
Warning: include/linux/hw_random.h:54 struct member 'cleanup_done' not
 described in 'hwrng'
Warning: include/linux/hw_random.h:54 struct member 'dying' not described
 in 'hwrng'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>