staging: unisys: fix visorchipset sysfs attribute functions

This patch cleans up the style, error handling, and string handling in the sysfs
functions recently added to visorchipset:

- Use of sscanf() was changed to type-appropriate kstrto*() functions
- error handling was simplified
- the error return value of visorchannel_write() was corrected
- switch use of driver-specific types to kernel types

Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>