IB/isert: Simplify signature cap check

Use if/else clause instead of "condition ? val1 : val2" to make the code
cleaner and simpler.

Link: https://lore.kernel.org/r/20210110111903.486681-3-mgurtovoy@nvidia.com
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>