i2c: rust: mark I2cAdapter methods as inline

When building the kernel using llvm-19.1.7-rust-1.85.0-x86_64, the
following symbols are generated:

$ nm vmlinux | grep ' _R'.*I2cAdapter | rustfilt
ffffffff817ff380 T <kernel::i2c::I2cAdapter>::get
ffffffff817ff400 T <kernel::i2c::I2cAdapter as kernel::sync::aref::AlwaysRefCounted>::dec_ref
ffffffff817ff3e0 T <kernel::i2c::I2cAdapter as kernel::sync::aref::AlwaysRefCounted>::inc_ref

However, these Rust symbols are trivial wrappers around the
`i2c_get_adapter` and `i2c_put_adapter` functions. It doesn't make sense
to go through a trivial wrapper for these functions.

Link: https://github.com/Rust-for-Linux/linux/issues/1145
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Nicolás Antinori <nico.antinori.7@gmail.com>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Reviewed-by: Igor Korotin <igor.korotin@linux.dev>
Signed-off-by: Igor Korotin <igor.korotin@linux.dev>