firmware: arm_scpi: drop unnecessary type cast to scpi_shared_mem

This patch drops the only present type cast of the SCPI payload pointer
to scpi_shared_mem inorder to align with other occurrences, IOW for
consistency.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>