spi: mtk_spim: prevent global pll clock override

With commit 793e6230
 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>