memory: brcmstb: dpfe: use byte 3 of registers MR4-MR8

For dual-rank LPDDR4, result data for any command is placed in byte 0
and byte 3 of the corresponding MR register by the firmware.
Single-rank RAM was supposed to work the same way. However, due to a
firmware bug, result values are only placed in byte 3 of the
corresponding MR register.

Since byte 3 works for single-rank and dual-rank setups, we change the
Linux driver to always use byte 3, thus returning the correct value in
either case.

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