drivers: flash: flash_rpi_pico: fix indentation and remove unused variables

Cleaned up the flash_rpi_pico driver to improve code readability and
compliance with Zephyr coding standards. Fixed inconsistent indentation
across the file and removed variables that were declared but not used.

A few improvements are credited to Manu3l0us.

Signed-off-by: Hanan Arshad <hananarshad619@gmail.com>