fix compiling errors of FATFileSystem when exFAT was enabled

[wdx04]

Summary of changes

Fixed compiling errors of FATFileSystem when exFAT was enabled(config "fat_chan.ff_fs_exfat" was set to true).
The return value of ff_memalloc() function should be explicitly converted into WCHAR* before assigning to variable "lfn".

Impact of changes

Migration actions required

Documentation

None.

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[0xc0170]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_unittests	✔️
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