USB: pl2303: Cleanup FISH and SOUP macros in pl2303 driver.

Replace the FISH and SOUP macros that violated the macro guidelines in CodingStyle.
Turn them into function calls with clearer variable names.

Signed-off-by: Sarah Sharp <saharabeara@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>