s390/pai: move enum definition to header file

Move enum definition to header file. This is done in preparation
for a follow on patch where this enum will be used in another source
file.
Also change the enum name from paiext_mode to paievt_mode
to indicate this enum is now used for several events.
Make naming consistent and rename PAI_MODE_COUNTER to PAI_MODE_COUNTING.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>