serial: mpc52xx: Use default serial core x_char handler

mpc52xx_uart_send_xchar() is _identical_ to the default serial core
x_char handling behavior in uart_send_xchar().

Remove mpc52xx_uart_send_xchar().

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>