video: fbdev: arcfb: remove redundant initialization of variable err

The variable err is being initialized with a value that is never
read, the assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721102608.42694-1-colin.king@canonical.com