dmaengine: stm32-dmamux: Simplify code and save a few bytes of memory

STM32_DMAMUX_MAX_DMA_REQUESTS is small (i.e. 32) and when the 'dma_inuse'
bitmap is allocated, there is already a check that 'dma_req' is <= this
limit.

So, there is no good reason to dynamically allocate this bitmap. This
just waste some memory and some cycles.

Use DECLARE_BITMAP with the maximum bitmap size instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/2d8c24359b2daa32ce0597a2949b7b2bebaf23de.1659211633.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Vinod Koul <vkoul@kernel.org>