mtd: rawnand: gpmi: Set WAIT_FOR_READY timeout based on program/erase times

06781a50 Fixes the calculation of the DEVICE_BUSY_TIMEOUT register
value from busy_timeout_cycles. busy_timeout_cycles is calculated wrong
though: It is calculated based on the maximum page read time, but the
timeout is also used for page write and block erase operations which
require orders of magnitude bigger timeouts.

Fix this by calculating busy_timeout_cycles from the maximum of
tBERS_max and tPROG_max.

This is for now the easiest and most obvious way to fix the driver.
There's room for improvements though: The NAND_OP_WAITRDY_INSTR tells us
the desired timeout for the current operation, so we could program the
timeout dynamically for each operation instead of setting a fixed
timeout. Also we could wire up the interrupt handler to actually detect
and forward timeouts occurred when waiting for the chip being ready.

As a sidenote I verified that the change in 06781a50 is really
correct. I wired up the interrupt han...