PL011: Fix a bug in the UART FIFO polling

Before attempting to write a character, the PL011 driver polls
the PL011_UARTFR_TXFF bit to know whether the UART FIFO is full.
However, the comparison with 1 was incorrect because
PL011_UARTFR_TXFF is not at bit 0. This patch fixes it.

Change-Id: If78892345bbdc8a5e4ae4a1b7159753c609681b0
Merge pull request #127 from sandrine-bailleux/sb/fix-pl011-fifo-polling

PL011: Fix a bug in the UART FIFO polling