drivers/spi: Re-factor spi_crop_chunk

spi_crop_chunk is a property of the SPI controller since it depends
upon the maximum transfer size that is supported by the
controller. Also, it is possible to implement this within spi-generic
layer by obtaining following parameters from the controller:

1. max_xfer_size: Maximum transfer size supported by the controller
(Size of 0 indicates invalid size, and unlimited transfer size is
indicated by UINT32_MAX.)

2. deduct_cmd_len: Whether cmd_len needs to be deducted from the
max_xfer_size to determine max data size that can be
transferred. (This is used by the amd boards.)

Change-Id: I81c199413f879c664682088e93bfa3f91c6a46e5
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19386
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Tested-by: coreboot org <coreboot.org@gmail.com>

src/soc/mediatek/mt8173/flash_controller.c

@@ -108,11 +108,6 @@ static int sector_erase(int offset)
 	return 0;
 }
 
-unsigned int spi_crop_chunk(unsigned int cmd_len, unsigned int buf_len)
-{
-	return min(65535, buf_len);
-}
-
 static int dma_read(u32 addr, u8 *buf, u32 len, uintptr_t dma_buf,
 		    size_t dma_buf_len)
 {