clk: at91: fix PERIPHERAL_MAX_SHIFT definition

Fix the PERIPHERAL_MAX_SHIFT definition (3 instead of 4) and adapt the
round_rate and set_rate logic accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: "Wu, Songjun" <Songjun.Wu@atmel.com>