altera_uart: Don't take spinlock in already protected functions

Don't take the port spinlock in uart functions where the serial core
already takes care of locking/unlocking them.

The code would actually lock up on architectures where spinlocks are
implemented (not the case on nios2 where this driver is primarily used
for now, thus this bug didn't trigger).

Also protect calling altera_uart_rx_chars/altera_uart_tx_chars in the
interrupt handler by the port spinlock.

Thanks to Ian Abbott for pointing these issues out.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>