pinctrl: mpfs-mssio: use correct regmap function to set bank voltage

regmap_assign_bits() is not the correct function to use for an RMW
operation, as it maps to regmap_set_bits() or regmap_clear_bits() and
the former will never zero a bit. Use regmap_update_bits() instead,
which will actually set the bank voltages to what have been requested.

CC: stable@vger.kernel.org
Fixes: 488d704ed7b7 ("pinctrl: add polarfire soc mssio pinctrl driver")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>