mmc: cavium: Remove redundant pointer check

Remove redundant mmc->card check reported by smatch:

drivers/mmc/host/cavium.c:694 cvm_mmc_dma_request()
	warn: variable dereferenced before check 'mmc->card' (see line 675)

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>