avr32: fix use of non-existing portnr variable in at32_map_usart()

This patch fixes the use of the non-existing portnr variable in
at32_map_usart() to use the provided line number instead. Typo was introduced
in commit 2b348e2f.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>