video: fbdev: atyfb: remove set but not used variable 'mach64RefFreq'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/video/fbdev/aty/mach64_gx.c: In function ‘aty_var_to_pll_8398’:
drivers/video/fbdev/aty/mach64_gx.c:621:36: warning: variable
‘mach64RefFreq’ set but not used [-Wunused-but-set-variable]

It is never used, and so can be removed.

Signed-off-by: yu kuai <yukuai3@huawei.com>
Cc: zhengbin <zhengbin13@huawei.com>
Cc: yi.zhang <yi.zhang@huawei.com>
[b.zolnierkie: minor patch summary fixup]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120063327.43548-1-yukuai3@huawei.com