ieee802154: fix faulty check in set_phy_params api

phy_set_csma_params has a redundant (and impossible) check for
"retries", found by smatch. The check was supposed to be for
frame_retries, but wasn't moved during development when
phy_set_frame_retries was introduced. Also, maxBE >= 3 as required by
the standard is not enforced.

Remove the redundant check, assure max_be >= 3 and check -1 <=
frame_retries <= 7 in the correct function.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>