acpi_ec: changes in communication with hardware

Short description of the changes:

• attempt to retry some commands for which it is possible (read, query)
• always make a short sleep before checking EC status in polled mode
• periodically poll EC status in interrupt mode
• change logic for detecting broken interrupt delivery and falling back to polled mode
• check that EC is ready for input before starting a new command, wait if necessary

This commit is based on the original patch by David Naylor.

PR: kern/150517
Submitted by: David Naylor <naylor.b.david@gmail.com>
Reviewed by: jkim
MFC after: 3 weeks