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.