atmel_serial: avoid stopping pdc during transmission

I found a problem related to losing data during pdc transmission in
atmel_serial: connect ttyS1 with ttyS2 using a loopback cable, send 30
byte of packet from one to the other and waiting for 30 byte.  On the
other side just read and echo the data received.

We always call atmel_tx_dma() from the tasklet regardless of what interrupt
triggered it.

Signed-off-by: michael <trimarchi@gandalf.sssup.it>
Acked-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>