brcmfmac: fix end of loop check (signedness bug)

The problem here is that we loop until "remained_buf_len" is less than
zero, but since it is unsigned, it never is.

"remained_buf_len" has to be large enough to hold the value from
"mgmt_ie_buf_len".  That variable is type u32, but it only holds small
values so I have changed to both variables to int.

Also I removed the bogus initialization from "mgmt_ie_buf_len" so that
GCC can detect if it is used unitialized.  I moved the declaration of
"remained_buf_len" closer to where it is used so it's easier to read.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>