atmel_serial: fix broken RX buffer allocation

Introduced by atmel_serial-split-the-interrupt-handler.patch.

Thanks to michael <trimarchi@gandalf.sssup.it> for spotting it.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>