The original logic had an integer to unsigned integer
conversion, plus a float-point math. While gcc should be
able to do the match at compile time, other compilers might
not do the same. Also, those produce the following warnings
with static code analyzers:

	drivers/media/dvb-frontends/tda10086.c:300 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:303 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:306 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:309 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:312 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:315 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:318 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:321 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:324 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.
	drivers/media/dvb-frontends/tda10086.c:327 tda10086_set_symbol_rate() warn: unsigned 'symbol_rate' is never less than zero.

Change the logic to declare the reference constant as unsigned
and to not use float point math.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>