nios2: use long for ssize_t

This is consistent with nios2-linux. And resolved the warning,

cmd_nvedit.c: In function `do_env_export':
cmd_nvedit.c:660: warning: size_t format, ssize_t arg (arg 3)

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>

arch/nios2/include/asm/posix_types.h

@@ -17,7 +17,7 @@ typedef unsigned short	__kernel_ipc_pid_t;
 typedef unsigned short	__kernel_uid_t;
 typedef unsigned short	__kernel_gid_t;
 typedef unsigned long	__kernel_size_t;
-typedef int		__kernel_ssize_t;
+typedef long		__kernel_ssize_t;
 typedef int		__kernel_ptrdiff_t;
 typedef long		__kernel_time_t;
 typedef long		__kernel_suseconds_t;