Calling clk_get() with NULL as the second argument will give us "ipg"
clock as a result. The actual clock feeding into the peripheral is "per"
and, depending on the SoC, "ipg" and "per" may be separated by a clock
divider, so querying "ipg"'s rate may not result in rate that does not
represent the actual peripheral clock rate.

Change the code to request "per" as our peripheral clock to avoid
aforementioned problem.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>