i2c: Staticize local functions in mxc i2c driver

Some functions in the MXC i2c driver were not static, fix this by
making them so.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Stefano Babic <sbabic@denx.de>