i2c: Use __weak instead of __attribute__((weak, alias))

i2c: Use __weak instead of __attribute__((weak, alias))

Use __weak from linux/compiler.h instead of __attribute__((weak, alias))
to define overridable function. This patch is intended as a cleanup patch
to bring some consistency into the code.

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