drivers: serial: atmel_sam: Fix api to work with modbus

This patch adds fixes to the api so that it behaves as expected.
1 - The irq_tx_ready now only returns true if the tx interrupt is
enabled.
2 - The irq_tx_complete now functions as expected and returns true
only once all characters have been transmitted

Signed-off-by: Marius Scholtz <mariuss@ricelectronics.com>