staging: fwserial: (coding-style) rewrite comparisons to NULL as "!fifo->data"

Rewrite multiple comparisons to NULL as "!fifo->data"
to fix the last coding style problems of this file.

Signed-off-by: Christian Colic <colic.christian@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>