mmc: atmel-sdhci: do not check clk_set_rate return value

clk_set_rate will return rate in case of success and zero in case of
error, however it can also return -ev, but it's an ulong function.
To avoid any issues, disregard the return value of this call.
In case this call actually fails, nothing much we can do anyway, but we
can at least try with the previous values (or DT assigned-clocks)

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>