libertas: fix errors in functions accessing module registers

Problems located in the two functions lbs_set_reg() and lbs_get_reg():
- The offset field of struct cmd_ds_reg_access was not filled in
- The test on the return code of lbs_cmd_with_response() in function
  lbs_get_reg() was inverted

Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: John W. Linville <linville@tuxdriver.com>