PCI: pcie-cadence-ep: Remove redundant variable mmc

Variable mmc is being assigned but is never used hence it is redundant
and can be removed.

Cleans up clang warning:
warning: variable 'mmc' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
[lorenzo.pieralisi@arm.com: reworked commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Alan Douglas <adouglas@cadence.com>