net/tap: fix icc build

net/tap: fix icc build

build error:
.../dpdk/drivers/net/tap/rte_eth_tap.c(598):
error #279: controlling expression is constant
	RTE_ASSERT(!"unsupported request type: must not happen");

Although RTE_ASSERT helps debugging this issue when assert enabled,
constant expression in assert means this path can be taken during
runtime and there is no protection against it when assert is disabled.

Adding error log and error return back, replacing RTE_ASSERT.

Fixes: 7748a4b44196 ("net/tap: add debug messages")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>