UBUNTU: SAUCE: (noup) USB: usb-storage: Make MMC support optional on ums-realtek

UBUNTU: SAUCE: (noup) USB: usb-storage: Make MMC support optional on ums-realtek

BugLink: https://bugs.launchpad.net/bugs/1806335
Hardware vendors may not pay the MMC royalty, so MMC support needs to be
disabled on Ubuntu pre-installed hardwares.

The current approach is to use an out-of-tree module to disable MMC
support, but it's hard to maintain and it disables secure boot.

Use a new knob "enable_mmc" to enable/disable mmc support, so
pre-installed systems can easily turn MMC off.

Realtek doesn't want this feature in mainline kernel, but they are okay
to have this in Ubuntu's kernel. So let's have it here.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>