staging: unisys: visorbus: clean up parser_string_get function

The function parser_string_get was writing its own strnlen function, use
the kernel version instead.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>