mfd: 88pm800: Enhance error handling for sub pages probe/remove

pm800_pages_init and pm800_pages_exit are called by pm800_probe.
Change the code to enhance error handling and remove unused code at
pm800_pages_init/exit and pm800_probe.

Signed-off-by: Yi Zhang <yizhang@marvell.com>
Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>