Devices based on the i2400m emit a "barker" (32 bit unsigned) when
they boot. This barker is used to select, in the firmware file image,
which header should be used to process the rest of the file.

This commit implements said support, completing the series started by
previous commits.

We modify the i2400m_fw_dnload() firmware loading path by adding a
call to i2400m_bcf_hdr_find() [new function], in which the right BCF
header [as listed in i2400m->fw_hdrs by i2400m_fw_check()] is
located. Then this header is fed to i2400m_dnload_init() and
i2400m_dnload_finalize().

The changes to i2400m_dnload_finalize() are smaller than they look;
they add the bcf_hdr argument and use that instead of bcf. Likewise in
i2400m_dnload_init().
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>