video: fbdev: uvesafb: Fix set but not used warning

Fix W=1 warning by deleting unused local variable.

v2:
  - Updated subject (Lee)

v3:
  - Return early in case of an error (Thomas)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Michal Januszewski <spock@gentoo.org>
Cc: linux-fbdev@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-5-sam@ravnborg.org