i2c: at91: Handle return value of clk_prepare_enable

clk_prepare_enable() can fail here and we must check its return value.
Also, add a missing clk_disable_unprepare().

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>