fbmem: remove redundant assignment to err

Variable err is initialized to a value that is never read and it
is re-assigned later.  The initialization is redundant and can
be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190624223724.13629-1-colin.king@canonical.com