greybus: uart: fix typo in defintion

Fixing needless redefinition of operation types in gbsim reveals this typo
GB_UART_TYPE_SET_BREAK -> GB_UART_TYPE_SEND_BREAK. This patch should be
applied in lock-step to the patch to gbsim 'gbsim/uart: remove unnecessary
redefinition of operation types' since gbsim does not contain the typo.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>