serial: sprd: Change 'int' to 'unsigned int'

The register offset value should be 'unsigned int' type.

Moreover, prefer 'unsigned int' to bare use of 'unsigned'.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>