mmc: atmel-mci: remove unused sg_len variable

We got a warning:

drivers/mmc/host/atmel-mci.c:1086:15: warning: variable 'sg_len' set but
not used [-Wunused-but-set-variable]

Ideally we should check to see if sg_len is zero but looking
into the code closely, I didn't find any possible to do that as
atmci_start_request didn't even deploy any error handling for
its host->prepare_data hook. So even we return error value for iflags
like what other host drivers did, for instance, sdhci and dwmmc, it still
need some extra work to improve the code.

Just remove it to silent the warning, although it isn't perfect.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>