staging: fbtft: core: set smem_len before fb_deferred_io_init call

In the Linux kernel, the following vulnerability has been resolved:

staging: fbtft: core: set smem_len before fb_deferred_io_init call

The fbtft_framebuffer_alloc() calls fb_deferred_io_init() before

initializing info->fix.smem_len. It is set to zero by the

framebuffer_alloc() function. It will trigger a WARN_ON() at the

start of fb_deferred_io_init() and the function will not do anything.