media: vivid-radio-rx: add a cast to avoid a warning

The logic at vivid_radio_rx_g_tuner() is producint an overflow
warning:

	drivers/media/platform/vivid/vivid-radio-rx.c:250 vivid_radio_rx_g_tuner() warn: potential negative subtraction from max '65535 - (__builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( == , , (0))))))) * 65535) / delta'

Add a cast to prevent that.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>