ARM: decompressor: factor out routine to obtain the inflated image size

Before adding another reference to the inflated image size, factor
out the slightly complicated way of loading the unaligned little-endian
constant from the end of the compressed data.

Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>