lib:platform: Fix sbi_getc return type.

As per the current SBI specification, sbi_getc should return
an int instead of char.

In case of FIFO is empty, return -1 as per the specification.

Reported-by: Sergi Granell <xerpi.g.12@gmail.com>
Suggested-by: Thadeu Lima de Souza Cascardo <cascardo@cascardo.eti.br>
Signed-off-by: Atish Patra <atish.patra@wdc.com>