plugins: odp: Cosmetic changes in the IPsec implementation

plugins: odp: Cosmetic changes in the IPsec implementation

1) renamed ipsec_api to enable_odp_ipsec, for consistency
2) added crude ASSERTs to make sure the configuration is sane
3) removed passing ipsec_api as argument, as this is a global flag now
4) reorder code - put ipsec initialization before putting
   workers in polling mode
5) remove not used recycle vector from esp_encrypt.c/esp_decrypt.c
6) change clib_error_return to clib_error in ipsec.c, as
   clib_error_return silently passes when 0 is the first argument

Change-Id: Id2c6da985e872f12c8409918fb9cc6113b486d10
Signed-off-by: Szymon Sliwa <[email protected]>