dsp: adsp-loader: Add check around bytes read from nvmem cell

nvmem_cell_read() return no of bytes read in 'len' variable and
allocates similiar bytes of memory for buffer. Copy only these
number of bytes from buf to avoid out of bounds issues.

Change-Id: I712e5058d15deba939b6a743fcf5bff33ac51f84
Signed-off-by: Ajit Pandey <ajitp@codeaurora.org>