power: supply: da9150-fg: remove VLA usage

In preparation to enabling -Wvla, remove VLA usage and replace it
with fixed-length arrays.

DA9150_QIF_LONG_SIZE (4 bytes) is the biggest size of an attribute which can
be accessed [1].

Fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

[1] https://marc.info/?l=kernel-hardening&m=152059600524753&w=2



Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>