enable BKOPS to manual mode

Check if the BKOPS is supported before enabling it and setting to manual
mode.

Tested:
Before
```
$ mmc extcsd read /dev/mmcblk0 | grep BKOPS
Background operations suppo

enable BKOPS to manual mode

Check if the BKOPS is supported before enabling it and setting to manual
mode.

Tested:
Before
```
$ mmc extcsd read /dev/mmcblk0 | grep BKOPS
Background operations support [BKOPS_SUPPORT: 0x01]
Background operations status [BKOPS_STATUS: 0x00]
Enable background operations handshake [BKOPS_EN]: 0x00
```

After
```
eStoraged[29228]: BKOPS is supported for /dev/mmcblk0
eStoraged[29228]: Successfully enable BKOPS for /dev/mmcblk0
```

```sh
$ mmc extcsd read /dev/mmcblk0 | grep BKOPS
Background operations support [BKOPS_SUPPORT: 0x01]
Background operations status [BKOPS_STATUS: 0x00]
Enable background operations handshake [BKOPS_EN]: 0x01
```

Change-Id: Id2f80310b4f420192205aebdbc7726bfaac0dc15
Signed-off-by: Willy Tu <wltu@google.com>

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