mmc_device_darwin: Use mmc intermediate variable

Instead of the complex cast in various places use a intermediate
variable for mmc, this makes the code a lot easier to read.
4 jobs for fix-darwin-mmc-device in 3 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #320610
amd64 debian
build-debian

00:01:34

passed #320611
macos
build-macos

00:01:40

passed #320613
win32 amd64 debian
build-win32

00:01:54

passed #320612
win64 amd64 debian
build-win64

00:03:04