Bluetooth: use DIV_ROUND_UP in suitable places in btmrvl_sdio

There are two places where DIV_ROUND_UP may be used. It makes code a bit
clearer.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>