i.MX6 UART: Remove imx_transmit_buffer()

Prevent imx_start_tx() calling imx_transmit_buffer()
when the TX FIFO is empty because this can cause uart_write_wakeup()
to be called which in turn has potential for calling imx_start_tx()
when bound to the HCI UART TTY. There is a small probability of
recursion within an interrupt context. The recursion deadlocks
due to attempting to acquire a port spin_lock twice.

The solution is to remove imx_transmit_buffer() and to retain that
functionality in imx_txint(). In PIO mode, imx_start_tx() is called
with data in the TX ring buffer (data is assumed to be there) and enables
the UART TX empty interrupt. When the TX empty interrupt causes imx_txint()
to execute, uart_write_wakeup() can be called to add more data to the
ring buffer. imx_start_tx() can no longer cause uart_write_wakeup() to
execute so there is no risk of recursion.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>