solx_devfs.c: fix gcc warnings about casting away const when reading data

solx_devfs.c: fix gcc warnings about casting away const when reading data

solx_devfs.c: In function `pci_device_solx_devfs_write':
solx_devfs.c:1085: warning: cast discards qualifiers from pointer target type
solx_devfs.c:1089: warning: cast discards qualifiers from pointer target type
solx_devfs.c:1093: warning: cast discards qualifiers from pointer target type
solx_devfs.c:1097: warning: cast discards qualifiers from pointer target type

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>