media: dvb-frontends/stv0910: read symbolrate in get_frontend()

Utilise get_cur_symbol_rate() in get_frontend() to update the
dtv_frontend_properties with the current symbol rate as reported by the
demodulator.

Reported-by: Richard Scobie <rascobie@slingshot.co.nz>
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>