drivers: flash: stm32 qspi 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.
The HAL_QSPI_Init() accepts Init.FlashSize where "FlashSize+1
is effectively the number of address bits
required to address the flash memory."
To get 64MBytes = 2^26, the value of the Init.FlashSize must be 25.
and bit written to the DCR = 25.

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