i2c: rcar: fix mask value of prohibited bit

According to documentation, Bit 7 of ICMSR is unused and 0 should be
written to it. Fix the mask accordingly.

Signed-off-by: Hiromitsu Yamasaki <hiromitsu.yamasaki.ym@renesas.com>
[wsa: edited commit message]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>