firmware: arm_scmi: Fix list protocols enumeration in the base protocol

firmware: arm_scmi: Fix list protocols enumeration in the base protocol

While enumerating protocols implemented by the SCMI platform using
BASE_DISCOVER_LIST_PROTOCOLS, the number of returned protocols is
currently validated in an improper way since the check employs a sum
between unsigned integers that could overflow and cause the check itself
to be silently bypassed if the returned value 'loop_num_ret' is big
enough.

Fix the validation avoiding the addition.

Link: https://lore.kernel.org/r/20220330150551.2573938-4-cristian.marussi@arm.com
Fixes: b6f20ff8bd94 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>