mtd: gpio_flash: handle case where offset + len exceeds the window size

Fix the bug in handling gpio flash read/write when offset + len
from MTD exceeds the window size

Signed-off-by: Aaron Wu <Aaron.wu@analog.com>
[Brian: made some commentary edits. Also note that the BUG_ON() was
provably false for all non-negative inputs (since x % y <= x), so we
dropped it.]

Signed-off-by: Brian Norris <computersforpeace@gmail.com>