tty/serial: at91: fix some macro definitions to fit coding style

This patch updates macro definitions in atmel_serial.h to fit the
80 column rule.

Please note that some deprecated comments such as "[AT91SAM9261 only]"
are removed as the corresponding bits also exist in some later chips.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>