fw_env: fix build error

The following error appears:

tools/env/fw_env.c:1149:25: error: lvalue required as unary ‘&’ operand
  rc = write(fd, &ENV_REDUND_OBSOLETE, sizeof(ENV_REDUND_OBSOLETE));

Fixes: d3716dd6 ("env: Rename the redundancy flags")

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Tested-by: Joris Offouga <offougajoris@gmail.com>
Tested-by: Heiko Schocher <hs@denx.de>
Suggested-by: Heiko Schocher <hs@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>