ethdev: get registers width

ethdev: get registers width

The ethtool app was allocating too little space for 64-bit
registers which resulted in memory corruption.

Removes hard-coded assumption that device registers
are always 32 bits wide. The rte_eth_dev_get_reg_length
and rte_eth_dev_get_reg_info callbacks did not
provide register size to the app in any way while is
needed to allocate correct number of bytes before
retrieving registers using rte_eth_dev_get_reg.

This commit changes rte_eth_dev_get_reg_info so that
it can be used to retrieve both the number of registers
and their width, and removes the now-redundant
rte_eth_dev_get_reg_length.

Signed-off-by: Zyta Szpak <zyta.szpak@semihalf.com>
Acked-by: Remy Horton <remy.horton@intel.com>