drivers: base: swnode: check if swnode is NULL before dereferencing it

The to_software_mode() macro can potentially return NULL, so also add
a NULL check on swnode before dereferencing it to avoid any NULL
pointer dereferences.

Detected by CoverityScan, CID#1476052 ("Explicit null dereferenced")

Fixes: 59abd836 (drivers: base: Introducing software nodes to the firmware node framework)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>