s390/ipl: cleanup macro usage

ipl.c uses 3 different macros to create a sysfs show function for ipl
attributes. Define IPL_ATTR_SHOW_FN which is used by all macros.

Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>