drivers: flash: stm32 ospi driver correct device size

The find_lsb_set is giving the position of the first '1' found,
starting from 1. "Bits are numbered starting at 1
from the least significant bit."
So that the find_lsb_set(64MBytes) is 27.
To get 64MBytes = 2^26, the value of the Init.DeviceSize must be 26.

Signed-off-by: Francois Ramu <francois.ramu@st.com>