spi: dw: fix potential variable assignment error

[shaftarger]

spi::mode is defined by framework for several SPI capabilities, such as polarity, phase, bit-endian, wire number.
Directly use this variable for setting controller's polarity and phase is inappropriate.
Since SPI framework has its definition, fetch required bit in spi::mode and set to register.

[KernelPRBot]

Hi @shaftarger!

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