iscsi_initiator: make logout_req::reason unsigned char

Previously this reported an error from Clang 3.9.0: implict conversion
from 'int' to 'char' changes value from 128 to -128.

Discussed with:	dim, trasz
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D7699

sys/dev/iscsi_initiator/iscsi.h

@@ -356,7 +356,7 @@ typedef struct text_req {
 
 typedef struct logout_req {
      char	cmd;	// 0x06
-     char	reason;	// 0 - close session
+     u_char	reason;	// 0 - close session
      			// 1 - close connection
      			// 2 - remove the connection for recovery
      char	_2[2];