mmc: core: Remove redundant null check

mmc: core: Remove redundant null check

This change removes a redundant null check found by Smatch.

Fixes: 403a0293f1c2 ("mmc: core: Add open-ended Ext memory addressing")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-mmc/345be6cd-f2f3-472e-a897-ca4b7c4cf826@stanley.mountain/
Signed-off-by: Avri Altman <avri.altman@sandisk.com>
Link: https://lore.kernel.org/r/20250319203642.778016-1-avri.altman@sandisk.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>