mtd/m25p80: retlen is never NULL

This is just a cleanup, it doesn't fix any bugs.

These functions all check retlen inconsistently and it generates a
warning in Smatch (http://smatch.sf.net

).  If retlen were ever NULL it
would cause an oops and the code has been this way since 2006 so someone
would have complained.  Also I looked at other places that implemented
the mtd read and write functions and they dereference retlen without
checking.

I removed the checks.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>