spi: mtk_spim: prevent global pll clock override

With commit 793e6230 ("spi: mtk_spim: get spi clk rate only once") a
new system to calculate the SPI clocks has been added.

Unfortunately, the do_div macro overrides the global priv->pll_clk_rate
field. This will cause to have a reduced clock rate on each subsequent
SPI call.

Signed-off-by: Valerio 'ftp21' Mancini <ftp21@ftp21.eu>
Signed-off-by: Nicolò Veronese <nicveronese@gmail.com>