fw_env: fix incorrect usage of open(O_CREAT)

fw_env: fix incorrect usage of open(O_CREAT)

When using open(), the O_CREAT flag must be given a mode, otherwise it
uses random garbage from the stack.  Also, it can fail to build:

In file included from /usr/include/fcntl.h:290:0,
                 from fw_env_main.c:42:
In function 'open',
    inlined from 'main' at fw_env_main.c:97:9:
/usr/include/bits/fcntl2.h:50:24: error: call to '__open_missing_mode' declared
	with attribute error: open with O_CREAT in second argument needs 3 arguments

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>