cxgb4: Fix build failure when CONFIG_TLS=m

After commit 9d2e5e9e ("cxgb4/ch_ktls: decrypted bit is not enough")
whenever CONFIG_TLS=m and CONFIG_CHELSIO_T4=y, the following build
failure occurs:

ld: drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.o: in function
`cxgb_select_queue':
cxgb4_main.c:(.text+0x2dac): undefined reference to `tls_validate_xmit_skb'

Fix this by ensuring that if TLS is set to be a module, CHELSIO_T4 will
also be compiled as a module. As otherwise the cxgb4 driver will not be
able to access TLS' symbols.

Fixes: 9d2e5e9e

 ("cxgb4/ch_ktls: decrypted bit is not enough")
Signed-off-by: Tom Seewald <tseewald@gmail.com>
Link: https://lore.kernel.org/r/20201120192528.615-1-tseewald@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>