ACPI: EC: Restore udelay in poll mode

This fixes keyboard event handling on some systems.

Note that this delay was thought unnecessary, and removed
from linux-2.6.20 with 50c1e1138cb94f6aca0f8555777edbcefe0324e2
'ACPI: ec: Drop udelay() from poll mode. Loop by reading status field instead.'

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>