cc2520: Enable hardware filtering all the time

This prevents false packets to get accepted/generated into RX fifo.
Also, this will make AUTOACK working properly, when enabled. For some
reasons FRMFILT1 and SRCMATCH need to get their reset values written at
initialization, or then hw filtering won't work. This behavior is not
documented as it seems.

Change-Id: Ic0fe664dbc3b17d85d794c12b77bdbaafeb601f5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>