video: fbdev: tdfx: Fix set but not used warning in att_outb()

The tmp variable was assigned but the result was never used,
so delete the tmp variable.

v2:
  - Update subject (Lee)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201128224114.1033617-13-sam@ravnborg.org