i2c: img-scb: use DIV_ROUND_UP to round divisor values

Using % can be slow depending on the architecture.

Using DIV_ROUND_UP is nicer and more efficient way to do it.

Fixes: commit 27bce457 ("i2c: img-scb: Add Imagination Technologies I2C SCB driver")
Signed-off-by: Sifan Naeem <sifan.naeem@imgtec.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>