clk: imx: pllv3: fix potential 'divide by zero' in av_get_rate()

Guard 'mfd==0' to prevent 'divide by zero' issue in
clk_pplv3_av_get_rate(). If it is 0, let's return with EIO since mfd
should never be 0 at all.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>