serial: serial_core.c: printk replacement

printk replaced with corresponding dev_* .
fixed two broken user-visible strings used by the corresponding printk.
the null check for uport->dev and port->dev is removed as dev_* will check for
null while printing.
printing of dev_name(uport->dev) and dev_name(port->dev) also removed as those
are being printed by dev_* .

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>