drivers: net: davicom: Fixed unused but set variable with W=1

drivers/net/ethernet/davicom//dm9000.c: In function ‘dm9000_dumpblk_8bit’:
drivers/net/ethernet/davicom//dm9000.c:235:6: warning: variable ‘tmp’ set but not used [-Wunused-but-set-variable]

The driver needs to read packet data from the device even when the
packet is known bad. There is no need to assign the data to a variable
during this discard operation.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>