driver: qspi: correct QSPI disable CS reset value

Correcting QSPI disable/unselect CS reset value.
CTRL_CORE_CONTROL_IO_2: QSPI_MEMMAPPED_CS[10:8]

This is not causing any issue, but its better
to untouch the reserved bits.

Praneeth Bajjuri <praneeth@ti.com>
Signed-off-by: Ravi Babu <ravibabu@ti.com>