s390/boot: change initial program check handler to disabled wait psw

The program check handler of the kernel image points to
startup_pgm_check_handler. However an early program check which happens
while loading the kernel image will jump to potentially random code, since
the code of the program check handler is not yet loaded; leading to a
program check loop.

Therefore initialize it to a disabled wait psw and let the startup code set
the proper psw when everything is in memory.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>