mmc: atmel-mci: fix bad variable type for clkdiv

clkdiv is declared as an u32 but it can be set to a negative value
causing a huge divisor value. Change its type to int to avoid this case.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: <stable@vger.kernel.org> # 3.4 and later
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>