i2c: at91: fix clk_offset for sam9x60

In SAM9X60 datasheet, FLEX_TWI_CWGR register description mentions clock
offset of 3 cycles (compared to 4 in eg. SAMA5D3).
This is the same offset as in SAMA5D2.

Fixes: b0027792 ("i2c: at91: add new platform support for sam9x60")
Suggested-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>