firmware: arm_ffa: Replace SCMI by FF-A in the macro

Commit 22779149 ("firmware: arm_ffa: Emit modalias for FF-A devices")
added modalias for FF-A devices. However the macro added used SCMI
incorrectly. Replace the reference to SCMI by FF-A and update the macro.
(i.e. s/SCMI/FFA/)

No functional change.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-1-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>