[POWERPC] boot: Simplify gunzip_finish

Call gunzip_partial to calculate the remaining length and copy the
data to the user buffer.  This makes it shorter and reduces
duplication.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>