memory/atmel-ebi: Fix ns <-> cycles conversions

at91sam9_ebi_get_config() is incorrectly converting timings in clock
cycles into timings in nanoseconds by multiplying the cycle values by
the clk rate instead of the clk period.

at91sam9_ebi_xslate_config() has the same problem for the
tdf_ns -> tdf_cycles conversion.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Chris Leahy <leahycm@gmail.com>
Fixes: 6a4ec4cd

 ("memory: add Atmel EBI (External Bus Interface) driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>