[media] drxj: don't do math if not needed

[media] drxj: don't do math if not needed

While there's no risk of divison by zero, the logic there is akward, as it
does the calculus for the numerator and denominator before checking
if this will be used.

Change the order to check first if the denominator is zero, and only
calculating the numerator/denominator if not.

This should also avoid those smatch errors:
	drivers/media/dvb-frontends/drx39xyj/drxj.c:9605 ctrl_get_qam_sig_quality() debug: sval_binop_unsigned: divide by zero
	drivers/media/dvb-frontends/drx39xyj/drxj.c:9605 ctrl_get_qam_sig_quality() debug: sval_binop_unsigned: divide by zero
	drivers/media/dvb-frontends/drx39xyj/drxj.c:9605 ctrl_get_qam_sig_quality() debug: sval_binop_unsigned: divide by zero
	drivers/media/dvb-frontends/drx39xyj/drxj.c:9605 ctrl_get_qam_sig_quality() debug: sval_binop_unsigned: divide by zero
	drivers/media/dvb-frontends/drx39xyj/drxj.c:9605 ctrl_get_qam_sig_quality() debug: sval_binop_unsigned: divide by zero

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>