stm32f3: rcc: consistent masks for pll multiplier

All other masks consistently used a separate mask/shift define, bring the pll
multiplier function in line, and use the same form as other functions.

lib/stm32/f3/rcc.c

@@ -368,8 +368,11 @@ void rcc_set_hpre(uint32_t hpre)
 
 void rcc_set_pll_multiplier(uint32_t pll)
 {
-	RCC_CFGR = (~RCC_CFGR_PLLMUL_MASK & RCC_CFGR) |
-		   (pll << RCC_CFGR_PLLMUL_SHIFT);
+	uint32_t reg32;
+
+	reg32 = RCC_CFGR;
+	reg32 &= ~(RCC_CFGR_PLLMUL_MASK << RCC_CFGR_PLLMUL_SHIFT);
+	RCC_CFGR = (reg32 | (pll << RCC_CFGR_PLLMUL_SHIFT));
 }