drivers: flash: Removed a __packed attribute from struct jesd216_bfp

The struct jesd216_bfp was declared as __packed but later in the code
the address of a member was given to a non-packed pointer, potentially
causing reading of wrong addresses, and causing warnings with the
IAR ICCARM compiler.

After studying the JEDEC documentation JESD216F.02, section
6.4.2 JEDEC Basic Flash Parameter Header: 2nd DWORD, the struct must
be aligned to 4 so __packed is not necessary, just 4 byte alignment.

Signed-off-by: Lars-Ove Karlsson <lars-ove.karlsson@iar.com>