pinctrl: sh-pfc: r8a77990: Fix MOD_SEL0 bit3 when using TX0

According to the R-Car Gen3 Hardware Manual Errata for Rev 1.00 of
Aug 24, 2018, the MOD_SEL0 bit3 is set to 0 when TX0_A pin function is
selected, and the MOD_SEL0 bit3 is set to 1 when TX0_B pin function is
selected.

Fixes: 6d4036a1 ("pinctrl: sh-pfc: Initial R8A77990 PFC support")
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>