usb: dwc3: dwc3-msm: Avoid overriding DBM_GEN_CFG register

The current register write to DBM_GEN_CFG to update the current speed
is overwriting the entire register.  Avoid this, as this also
disables ZLT support on DBM.  During validation of QDSS, a QDSS data
stall was seen, as the QDSS BAM generated a ZLT, while DBM did not
enable it.  DBM ZLT support is recommended to be enabled in the HPG.

Change-Id: Ieef4a42d3ac6f2a836a5c25530492c95669047b0
Signed-off-by: Wesley Cheng <wcheng@codeaurora.org>