spi: ti_qspi: Fix failure on multiple READ_ID cmd

Populating QSPI_RD_SNGL bit(0x1) in priv->cmd means that value
QSPI_INVAL (0x4) is not written to CMD field of QSPI_SPI_CMD_REG in
ti_qspi_cs_deactivate(). Therefore CS is never deactivated between
successive READ ID which results in sf probe to fail.
Fix this by not populating priv->cmd with QSPI_RD_SNGL and OR it wih
priv->cmd as required (similar to the convention followed in the
driver).

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>