lib: Split off fpi_ssm functions

Rename drv.c to something more fitting to its contents, and move state
machine code to its own header.

libfprint/meson.build

@@ -4,7 +4,8 @@ libfprint_sources = [
     'async.c',
     'core.c',
     'data.c',
-    'drv.c',
+    'fpi-ssm.c',
+    'fpi-ssm.h',
     'img.c',
     'imgdev.c',
     'poll.c',