firmware: arm_ffa: Explicitly cast return value from FFA_VERSION before comparison

The return value ver.a0 is unsigned long type and FFA_RET_NOT_SUPPORTED
is a negative value.

Since the return value from the firmware can be just 32-bit even on
64-bit systems as FFA specification mentions it as int32 error code in
w0 register, explicitly casting to s32 ensures correct sign interpretation
when comparing against a signed error code FFA_RET_NOT_SUPPORTED.

Without casting, comparison between unsigned long and a negative
constant could lead to unintended results due to type promotions.

Fixes: 3bbfe987 ("firmware: arm_ffa: Add initial Arm FFA driver support")
Reported-by: Andrei Homescu <ahomescu@google.com>
Message-Id: <20250221095633.506678-1-sudeep.holla@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>