connector: Fix invalid conversion in cn_proc.h

The implicit conversion from unsigned int to enum
proc_cn_event is invalid, so explicitly cast it
for compilation in a C++ compiler.
/usr/include/linux/cn_proc.h: In function 'proc_cn_event valid_event(proc_cn_event)':
/usr/include/linux/cn_proc.h:72:17: error: invalid conversion from 'unsigned int' to 'proc_cn_event' [-fpermissive]
   72 |         ev_type &= PROC_EVENT_ALL;
      |                 ^
      |                 |
      |                 unsigned int

Signed-off-by: Matt Jan <zoo868e@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>