mmc: core: enable BKOPS by read-modify-write instead of override

This change adds a read-modify-write logic to BKOPS feature enable.
It is required in order to avoid overriding other fields defined
in BKOPS_EN register.

Change-Id: I689f5cd14d9ec1bb881f503a0418026a59e6c197
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>