cc2520: Fix how FREGs and SREGs are accessed

- SREGs should use MEMRG/MEMWR instructions, FREGs use REGRD/REGWR
- REGRD/REGWR get the register address in the first byte along the
  instruction.

Reported-by: Igor Modino <igor@emutex.com>

Change-Id: I0da9de5ecd4cab914c8711bd4276e7a2bc6147a5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>