fs: fix clippy warning

[00xc]

Fix a new clippy warning that comes up with the latest nightly toolchain:

warning: this argument is a mutable reference, but not used mutably
   --> src/fs/filesystem.rs:265:36
    |
265 | pub fn write(fh: &FileHandle, buf: &mut [u8]) -> Result<usize, SvsmError> {
    |                                    ^^^^^^^^^ help: consider changing to: `&[u8]`
    |
    = warning: changing this function will impact semver compatibility
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
    = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default

[roy-hopkins]

LGTM