net/smc: Refactor smc ism v2 capability handling

Encapsulate the smc ism v2 capability boolean value
in a function for better information hiding.

Signed-off-by: Guvenc Gulce <guvenc@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>