drivers: flash: stm32l5_u5: refactor flash_stm32_page_layout() for clarity

While diagnosing a problem on the STM32U585 in this function in
flash_stm32l5_u5.c, I had difficulty sussing out the flow of the
function and conditional logic.

This refactor seeks to improve clarity through a simplified flow control
using an early return to short circuit logic on subsequent calls to the
function and slightly more thorough comments.

Tested using tests/drivers/flash on the b_u585i_iot02a and in a
proprietary application and board which uses the STM32U585 and littlefs.

Signed-off-by: Peter Maxwell Warasila <madmaxwell@soundcomesout.com>