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>