usb: musb: make use_sg flag URB specific

Since highmem PIO URB handling was introduced in:

8e8a5516 usb: musb: host: Handle highmem in PIO mode

when a URB is being handled it may happen that the static use_sg flag
was set by a previous URB with buffer in highmem.  This leads to error
in handling the present URB.

Fix this by making the use_sg flag URB specific.

Cc: stable <stable@vger.kernel.org> # 3.7+
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>