Fix crash when device doesn’t have write permission

[linkmauve]

Prevents this panic on launch of the host example:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Access', src/lib.rs:51:61

[coveralls]

Pull Request Test Coverage Report for Build 786732826

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• 115 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+12.2%) to 45.137%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
../../../../../usr/share/rust/.cargo/registry/src/github.com-1ecc6299db9ec823/rusb-0.6.5/src/device.rs	11	0.0%
src/lib.rs	16	55.86%
src/device.rs	88	56.76%

Totals
Change from base Build 351851736:	12.2%
Covered Lines:	543
Relevant Lines:	1203

---

💛 - Coveralls