memory: brcmstb: dpfe: Compute checksum at __send_command() time

Instead of pre-computing the checksum, do it at the time we send the
command, this reduces the possibility of introducing errors as well as
limits the amount of code necessary while adding new commands and/or new
API versions. The MSG_CHKSUM enumeration value is no longer necessary
and is removed.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Markus Mayer <mmayer@broadcom.com>