[sw, dif_uart] Fix use of wrong enum in return code
[sw, dif_uart] Fix use of wrong enum in return code

Two functions were returning enum values from the wrong enum type. The
numerical value was the same as for the correct enum, so this was only a
latent bug. This was noticed when compiling with Clang, which warns about
the use of mismatching enums.

Signed-off-by: Luís Marques <3b3ecfbddc4b81704f7616abd2273612f2861878@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan