i2c-designware: Remove an useless local variable "num"

We couldn't know the original intent for this variable, but at this
point it's useless.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>