ixgbevf: remove redundant initialization of variable 'dma'

Variable dma is initialized with a value that is never read, later
on it is re-assigned a new value, hence the initialization is redundant
and can be removed.

Cleans up clang warning:
drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c:584:13: warning: Value
stored to 'dma' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>