s390/boot: Use boot_printk() instead of sclp_early_printk()

Consistently use boot_printk() everywhere instead of sclp_early_printk() at
some places. For some places it was required (e.g. als.c), in order to stay
in code compiled for the same architecture level, for other places it is
not obvious why sclp_early_printk() was used instead of
decompressor_printk().

Given that the whole decompressor code is compiled for the same
architecture level, there is no requirement left to use different
printk functions.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>