Only do bank calculation on STM32L4 devices with dual banked flash (#751)

* Only do bank calculatio on SRM32L4 devices with dual banked flash

RM0394 covers the STM32L41xx, 42xx, 43xx, 44xx, 45xx, and 46xx. These
devices are all employ single banked flash and have chip id's
of 0x464 for the 41xx/42xx, 0x435 for 43xx/44xx, and 0x462 for 45xx/46xx
It's also worth noting that bit 21 of the FLASH_OPTR register is marked
as resevred for these chips, and isn't an indicator of dual banked
flash.

RM0392 covers the STM32L4x1, cpu_id 0x415 and can be dual banked.

RM0351 covers the STM32L4x5/4x6, cpu_ids 0x415 & 0x461 and can be dual
banked

RM0432 covers the STM32L4Rx/4Sx, cpu_id 0x470 and can be dual banked.

This PR modifies the calculate_L4_page functio to only factor bank
calculations for the devices above which can support dual banked flash.

* Converted tabs to spaces on added line