hw/net/e1000e: Fix compiler warning

slave:~/.xie/qemu-colo # gcc --version
gcc (SUSE Linux) 4.3.4 [gcc-4_3-branch revision 152973]

slave:~/.xie/qemu-colo # make -j8
CC    hw/net/e1000e_core.o
hw/net/e1000e_core.c:56: warning: ‘e1000e_set_interrupt_cause’ declared inline after being called
hw/net/e1000e_core.c:56: warning: previous declaration of ‘e1000e_set_interrupt_cause’ was here
LINK  x86_64-softmmu/qemu-system-x86_64

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>