serial: sirf: move to writel for TXFIFO instead of writeb

All SiRFSoC UART registers are in 32-bits. If we use writeb for
TXFIFO, actually all of 32-bits are still written, for TXTIFO,
only low 8-bits are valid, so in prima2&atlas6, this causes no
problem.
But in the new atlas7, using writeb to write UART registers will
cause an imprecise data abort as HW does check the "wrong" writeb.
So move to writel and this also makes the code consistent with
sirfsoc_uart_pio_tx_chars() in which we use writel.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>