usb: dw: Protect FIFO writes

Protect FIFO writes against other USB register accesses.
According to "DesignWare Cores USB 1.1/2.0 Device Subsystem-AHB/VCI Databook":
"During FIFO access, the application must not access the UDC/Subsystem
registers or vendor registers (for ULPI mode). After starting to access a
FIFO, the application must complete the transaction before accessing the
register."

Change-Id: I2aed833acaa2970310b6fd48f41a95bee2182ec1
Signed-off-by: Adrian Bradianu <adrian.bradianu@windriver.com>