mfd: cros_ec: Use a zero-length array for command data

mfd: cros_ec: Use a zero-length array for command data

Commit 1b84f2a4cd4a ("mfd: cros_ec: Use fixed size arrays to transfer
data with the EC") modified the struct cros_ec_command fields to not
use pointers for the input and output buffers and use fixed length
arrays instead.

This change was made because the cros_ec ioctl API uses that struct
cros_ec_command to allow user-space to send commands to the EC and
to get data from the EC. So using pointers made the API not 64-bit
safe. Unfortunately this approach was not flexible enough for all
the use-cases since there may be a need to send larger commands
on newer versions of the EC command protocol.

So to avoid to choose a constant length that it may be too big for
most commands and thus wasting memory and CPU cycles on copy from
and to user-space or having a size that is too small for some big
commands, use a zero-length array that is both 64-bit safe and
flexible. The same buffer is used for both output and input data
so the maximum of these values should be used to allocate it.

Suggested-by: Gwendal Grignou <[email protected]>
Signed-off-by: Javier Martinez Canillas <[email protected]>
Tested-by: Heiko Stuebner <[email protected]>
Acked-by: Lee Jones <[email protected]>
Acked-by: Olof Johansson <[email protected]>
Signed-off-by: Lee Jones <[email protected]>