The datasheet of is25wp256 says it supports SFDP. Get rid of the static
initialization of the flash parameters and init them when parsing SFDP.
Testing showed the flash using SPINOR_OP_READ_1_1_4_4B 0x6c,
SPINOR_OP_PP_4B 0x12 and SPINOR_OP_BE_4K_4B 0x21 before enabling SFDP.
After this patch, it parses the SFDP information and still uses the
same opcodes.
Set sector_size and n_sectors to zero as they will be discovered when
parsing SFDP.