drivers/net/e1000_spi.c: Fix build warnings

drivers/net/e1000_spi.c: Fix build warnings

Fix:
e1000_spi.c: In function 'spi_free_slave':
e1000_spi.c:115: warning: unused variable 'hw'
e1000_spi.c: In function 'do_e1000_spi':
e1000_spi.c:472: warning: 'checksum' may be used uninitialized in this function
e1000_spi.c:472: note: 'checksum' was declared here

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Acked-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>