atmel_serial: fix uart/console concurrent access

Strange chars appear on the serial port when a printk and a printf
happens at the same time.  This is caused by the pdc sending chars while
atmel_console_write (called from printk) is executing

Concurent access of uart and console to the same port leads to corrupted
data to be transmitted, so disable tx dma (PDC) while writing to the
console.

Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>