dma: stm32f4x: Fix warning when !SYS_LOG_INF

The following warning occurs when !SYS_LOG_INF due to the fact
that these variables are only used for information purposes.
When logging is <INFO then the variables aren't utilised at all.
To fix this we're removing the variables completely and using
the direct calls from within the information prints instead.

drivers/dma/dma_stm32f4x.c: In function 'dma_stm32_dump_reg':
drivers/dma/dma_stm32f4x.c:196:11: warning: unused variable 'sfcr' [-Wunused-variable]
  uint32_t sfcr  = dma_stm32_read(ddata, DMA_STM32_SFCR(id));
           ^
drivers/dma/dma_stm32f4x.c:195:11: warning: unused variable 'sm1ar' [-Wunused-variable]
  uint32_t sm1ar = dma_stm32_read(ddata, DMA_STM32_SM1AR(id));
           ^
drivers/dma/dma_stm32f4x.c:194:11: warning: unused variable 'sm0ar' [-Wunused-variable]
  uint32_t sm0ar = dma_stm32_read(ddata, DMA_STM32_SM0AR(id));
           ^
drivers/dma/dma_stm32f4x.c:193:11: warning: unused variable 'spar' [-Wunused-variable]
  uint32_t spar  = dma_stm32_read(ddata, DMA_STM32_SPAR(id));
           ^
drivers/dma/dma_stm32f4x.c:192:11: warning: unused variable 'ndtr' [-Wunused-variable]
  uint32_t ndtr  = dma_stm32_read(ddata, DMA_STM32_SNDTR(id));
           ^
drivers/dma/dma_stm32f4x.c:191:11: warning: unused variable 'scr' [-Wunused-variable]
  uint32_t scr   = dma_stm32_read(ddata, DMA_STM32_SCR(id));

Change-Id: I91a0373ef6c9afa8a342181c0ab24bd58743300d
Signed-off-by: Lee Jones <lee.jones@linaro.org>