msm: msm_bus: Fix inadvertent stomp on BKE_EN register

While setting the QoS mode of fixed mode masters bad masking
operations is causing accidental stomp on the upper nibble of the
BKE_EN register.

Change-Id: I5675e1dbc6fd5a01faac49c903e12a9c95e32254
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>