V4L/DVB (4533): Tda9887: add configuration setting for L standard PLL gating

Add a tuner config parameter for TDA9887, default_pll_gating_18, that
changes the L standard PLL gating value from 36% to 0% (datasheet says
0%, tda9887 code says 18%).
Turn this on for Microtune 4049FM5, as recomended by tuner datasheet.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>