memory: brcmstb_dpfe: fix testing array offset after use

Code should first check for valid value of array offset, then use it as
the index.  Fixes smatch warning:

  drivers/memory/brcmstb_dpfe.c:443 __send_command() error: testing array offset 'cmd' after use.

Fixes: 2f330caf ("memory: brcmstb: Add driver for DPFE")
Acked-by: Markus Mayer <mmayer@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20230513112931.176066-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>