firmware: arm_ffa: Remove unused 'compat_version' variable

firmware: arm_ffa: Remove unused 'compat_version' variable

The newly added ffa_compatible_version_find() function causes a
build warning because of a variable that is never used:

drivers/firmware/arm_ffa/driver.c:180:6: error: unused variable 'compat_version' [-Werror,-Wunused-variable]
        u32 compat_version;

Link: https://lore.kernel.org/r/20211026083400.3444946-1-arnd@kernel.org
Fixes: 8e3f9da608f1 ("firmware: arm_ffa: Handle compatibility with different firmware versions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>