media: drxj: better handle errors

as reported by smatch:
	drivers/media/dvb-frontends/drx39xyj/drxj.c:2157 drxj_dap_atomic_read_write_block() error: uninitialized symbol 'word'.

The driver doesn't check if a read error occurred. Add such
check.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>