usb: dwc3-msm: Don't use devm_ioremap when ringing GSI DB

Each time gsi_ring_db() is invoked, devm_ioremap_nocache() is
called to map the physical IO address in order to use writel().
However, devm_* calls will allocate a pointer to store with
the struct device and will not get freed until the device or
driver is removed, so in effect causes memory to be slowly
eaten for every GSI doorbell update, such as function bind.

Change the devm_ call to ioremap_nocache() followed by explicit
iounmap(). Also, if ioremap() fails, print an error message
and bail since the writel() would have no chance of succeeding.

Change-Id: I055774c24e2fbe86422756ea34b6a6e34ec21fb6
Signed-off-by: Jack Pham <jackp@codeaurora.org>