net: vxge: avoid unused function warnings

When CONFIG_PCI_MSI is disabled, we get warnings about unused functions
in the vxge driver:

drivers/net/ethernet/neterion/vxge/vxge-main.c:2121:13: warning: 'adaptive_coalesce_tx_interrupts' defined but not used [-Wunused-function]
drivers/net/ethernet/neterion/vxge/vxge-main.c:2149:13: warning: 'adaptive_coalesce_rx_interrupts' defined but not used [-Wunused-function]

We could add another #ifdef here, but it's nicer to avoid those warnings
for good by converting the existing #ifdef to if(IS_ENABLED()), which has
the same effect but provides better compile-time coverage in general,
and lets the compiler understand better when the function is intentionally
unused.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>