wimax/i2400m: be smarter about copying command buffer to bm_cmd_buf

Because some underlying bus APIs (like USB) don't like data buffers in
the stack or vmalloced areas, the i2400m driver provides a scratch
buffer (i2400m->bm_cmd_buf) for said low-level drivers to copy command
data to before passing it to said API. This is only used during boot
mode.

However, at some the code was copying the buffer even when the command
was already specified in said buffer. This is ok, but it needs to be
more careful. As thus, change so that:

(a) the copy happens only if command buffer is not the scratch buffer

(b) use memmove() in case there is overlapping

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>