HSI: Remove struct hsi_client private fields from kernel-doc

The kernel-doc how to says that structure fields that are inside a
"private:" area shouldn't be listed in the generated documentation
but the private fields for struct hsi_client private are listed.

This also fixes the following make htmldocs warnings:

.//include/linux/hsi/hsi.h:150: warning: Excess struct/union/enum/typedef member 'e_handler' description in 'hsi_client'
.//include/linux/hsi/hsi.h:150: warning: Excess struct/union/enum/typedef member 'pclaimed' description in 'hsi_client'
.//include/linux/hsi/hsi.h:150: warning: Excess struct/union/enum/typedef member 'nb' description in 'hsi_client'

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>