s390/decompressor: rename entry point to startup_decompressor

Rename the decompressor entry point to startup_decompressor to
avoid confusion, leaving startup_continue as the entry point of the
uncompressed image.

Also remove obsolete comment, as the decompressor code is
unconditionally called from boot/head.S now.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>