Fix UART TX FIFO thresholds
HAL_UARTFIFOSize was incorrectly returning the TX FIFO size, instead of the number of characters that can be written when the TX empty IRQ is received. This was causing data loss when DualSerial drives the interface at high speed (> 19200 baud) (OMAP3530), or data aborts when writing to UART_THR (DM3750, some kind of hardware quirk when the FIFO is full?)
Fix this by programming a sensible trigger level at startup, and returning the correct value from HAL_UARTFIFOSize.