firmware: rename struct firmware_buf to struct fw_priv

This reflects much better what this is used for. It also puts emphasis
on the fact we can and should be able to extend this data structure as
we see fit internally as its the opaque private pointer on struct
firmware.

As we rename the data structure, also rename a few functions that use it
to reflect better what they are for.
Signed-off-by: NLuis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>