wimax/i2400m: change the bcf_len to exclude the extended header size

The actual fw->size may not equal to the bcf size indicated in
the bcf header if the extended bcf debug header is added in the tail.

To reflect the actual fw size that will be downloaded to the device,
it is now retrived from from the size field indicated in the bcf header.

All of the headers (if there are extended headers) should indicate same
value for the size field since only one set of firmware chunks is downloaded

Signed-off-by: Cindy H Kao <cindy.h.kao@intel.com>