msm: msm_bus: Prevent BKE swtich from overwriting other fields

Incorrect bit mask could lead to other register fields getting
overwritten when doing a BKE bit switch.

Change-Id: I2cd7717eca48166dcc8cdb7ad855cc87e04c233b
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>