fs: exfat: Remove pointless variable uoffset

fs: exfat: Remove pointless variable uoffset

In exfat_generic_pread and exfat_generic_pwrite offset is passed in as a
off_t type which is defined as 'unsigned long long' so there is no need
to create the variable uoffset as a uint64_t as this is just a direct
copy of offset. Also remove the impossible test of 'offset < 0' as this
is always false due to offset being unsigned.

This issue found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>