usb: dwc3-msm: Remove XHCI_REV bit from newer cores

Newer DWC3 cores starting with revision 2.50a only support xHCI
revision 1.0. Hence the corresponding QSCRATCH register bit to
specify this is removed, and should only be set for older targets.
Also change the write operation to dwc3_msm_write_reg_field()
in order to not disturb the other bits in the register.

Change-Id: I31283ab491637820b1f26c6e8de95a1ee9bcc58c
Signed-off-by: Jack Pham <jackp@codeaurora.org>