spi-nor: intel-spi: Rename swseq to swseq_reg in 'struct intel_spi'

The ispi->swseq is used for register access. Let's rename it to
swseq_reg to better describe its usage.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>