rtc: m41t80: remove unneeded checks from m41t80_sqw_set_rate

rtc: m41t80: remove unneeded checks from m41t80_sqw_set_rate

m41t80_sqw_set_rate will be called with the result from
m41t80_sqw_round_rate, so might as well make
m41t80_sqw_set_rate(n) same as
m41t80_sqw_set_rate(m41t80_sqw_round_rate(n))

As Russell King wrote[1],
"clk_round_rate() is supposed to tell you what you end up with if you
ask clk_set_rate() to set the exact same value you passed in - but
clk_round_rate() won't modify the hardware."

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2012-January/080175.html

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>