net: ti: prueth_core: clean up related to return types of functions

Many functions such as prueth_hostconfig(), prueth_hostinit() doesn't
return any error code and can be void. Fix the return type to void
in such cases. Also check for return type of prueth_sw_emac_config()
since the function has a return value.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>