firmware: add helper to copy built-in data to pre-alloc buffer

This makes it clearer that the parameters passed are only used for
the preallocated buffer option, ie, when a caller uses:

	request_firmware_into_buf()

Otherwise this code won't run. We flip the logic just so the actual
prellocated buf code is not indented.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>