cmd: avb: remove mmc naming from generic block code

From: Andrii Chepurnyi <andrii_chepurnyi@epam.com>

Part of avb code uses mmc notation, but in fact
it uses generic block functions.

Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>