rust: pci: use to_result() in enable_device_mem()

Simplify enable_device_mem() by using to_result() to handle the return
value of the corresponding FFI call.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250314160932.100165-2-dakr@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>