i2c: at91: Read all available bytes at once

With FIFO enabled it is possible to read multiple bytes
at once in the interrupt handler as long as RXRDY is
set. This may also reduce the number of interrupts.

This patch polls RXRDY and reads all available bytes at
once.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[wsa: reformatted comment]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>