QSEECOM sets the Full Disk Encryption(FDE) key into
slot 0 by default, but for File Based Encryption(FBE)
device where Metadata Encryption(ME) is enabled, slot zero
is already used. So, an ioctl for setting FDE key into
Inline Crypto Engine(ICE) when called on device with ME
enabled, would overwrite the key set for ME, and hence
would lead to failure during reboot. Hence, set the slot
value of FDE key as 31 which is the last slot supported.
Use the same slot during read/write operations as well.

Tests: 1. Device bootup for sm8150 target with FDE enabled
       2. OTA update of kernel images with fix included.

Change-Id: Id77c4e55ce5740fd8babdfa690d3c39d39258f6e
Signed-off-by: Vaibhav Agrawal <vagrawa@codeaurora.org>