Use add_hh rather than add_hhh for ecc_mul_a_eh.

* ecc-mul-a-eh.c (ecc_mul_a_eh) [ECC_MUL_A_EH_WBITS == 0]: Use
add_hh rather than add_hhh.
(table_init) [[ECC_MUL_A_EH_WBITS > 0]: Likewise.
* ecc-internal.h (ECC_MUL_A_EH_ITCH) [ECC_MUL_A_EH_WBITS == 0]:
Reduced from 13*n to 12*n.