drivers: spi: fix the bug of slave selection in spi_dw

According to data sheet of dw_spi, ser reg is used to
select spi device/slave.

one bit in ser maps to one spi device/slave, i.e..
cs 0 ---> bit 0
cs 1 ---> bit 1
cs 2 ---> bit 2

the original code has a bug. the config->slave cannot
directly be written into ser. It should be mapped to
the correct bits through 1 << config->slave.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>