net/smc: remove constant and introduce helper to check for a pnet id

Use the existing symbol _S instead of SMC_ASCII_BLANK, and introduce a
helper to check if a pnetid is set. No functional change.

Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>