tty: serial: add dependence for CONFIG_SERIAL_FSL_LINFLEXUART

tty: serial: add dependence for CONFIG_SERIAL_FSL_LINFLEXUART

When CONFIG_SERIAL_FSL_LINFLEXUART=y and CONFIG_PRINTK is not set,
one compilation error is found as below:
drivers/tty/serial/fsl_linflexuart.c: In function linflex_earlycon_putchar:
drivers/tty/serial/fsl_linflexuart.c:608:31: error: CONFIG_LOG_BUF_SHIFT undeclared
(first use in this function); did you mean CONFIG_ISA_BUS_API?
  if (earlycon_buf.len >= 1 << CONFIG_LOG_BUF_SHIFT)
                               ^~~~~~~~~~~~~~~~~~~~
                               CONFIG_ISA_BUS_API
This because CONFIG_LOG_BUF_SHIFT is depended on CONFIG_PRINTK, fix this
by adding dependence for CONFIG_SERIAL_FSL_LINFLEXUART.

Fixes: b953815b819b ("tty: serial: Add linflexuart driver for S32V234")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Link: https://lore.kernel.org/r/20190820124015.28409-1-maowenan@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>