ACPI / EC: Remove unused functions and add prototype declaration in internal.h

Adds the prototype declarations of functions acpi_ec_add_query_handler()
and acpi_ec_remove_query_handler() in header file internal.h and removes
unused functions ec_burst_enable() and ec_burst_disable() in ec.c.

This eliminates the following warnings in ec.c:
drivers/acpi/ec.c:393:5: warning: no previous prototype for ‘ec_burst_enable’ [-Wmissing-prototypes]
drivers/acpi/ec.c:402:5: warning: no previous prototype for ‘ec_burst_disable’ [-Wmissing-prototypes]
drivers/acpi/ec.c:531:5: warning: no previous prototype for ‘acpi_ec_add_query_handler’ [-Wmissing-prototypes]
drivers/acpi/ec.c:552:6: warning: no previous prototype for ‘acpi_ec_remove_query_handler’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>