mtd: rawnand: gpmi: Fix setting busy timeout setting

The DEVICE_BUSY_TIMEOUT value is described in the Reference Manual as:

| Timeout waiting for NAND Ready/Busy or ATA IRQ. Used in WAIT_FOR_READY
| mode. This value is the number of GPMI_CLK cycles multiplied by 4096.

So instead of multiplying the value in cycles with 4096, we have to
divide it by that value. Use DIV_ROUND_UP to make sure we are on the
safe side, especially when the calculated value in cycles is smaller
than 4096 as typically the case.

This bug likely never triggered because any timeout != 0 usually will
do. In my case the busy timeout in cycles was originally calculated as
2408, which multiplied with 4096 is 0x968000. The lower 16 bits were
taken for the 16 bit wide register field, so the register value was
0x8000. With 2970bf5a ("mtd: rawnand: gpmi: fix controller timings
setting") however the value in cycles became 2384, which multiplied
with 4096 is 0x950000. The lower 16 bit are 0x0 now resulting in an
intermediate timeout whe...