mtd: spi-nor: issi: is25wp256: Init flash based on SFDP

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.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@sifive.com>
[tudor.ambarus@microchip.com: set sector_size and n_sectors to zero]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20220920184808.44876-1-sudip.mukherjee@sifive.com