mxc_spi: Round up clock divider

mxc_spi: Round up clock divider

Since the input frequency of the API is a maximum that should not be exceeded in
order for the devices to operate properly, the SPI clock divider should be
rounded up, not truncated.

Signed-off-by: Benoît Thébaudeau <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Stefano Babic <[email protected]>
Acked-by: Stefano Babic <[email protected]>