scsi: ufs: allow updating bBootLunEn via ioctl

"bBootLunEn" attribute value decides the active Boot LUN but there could
be a case where userspace may want to change it during image update.
This change allows writing to this parameter.

Change-Id: I6e8d630606d8caf6197ef662003ec1ee646afe81
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>