samples: cdc_acm_composite: Use proper type for variable 'read'

Function uart_fifo_read() returns 'int' (it may return negative values
in error cases). Its return value is stored in the variable 'read' that
is, however, of type 'size_t'.

Change the type of the variable 'read' from 'size_t' to 'int' to
accomodate proper handling of uart_fifo_read() invocation.

Coverity-CID: 248408

Signed-off-by: Aleksandar Markovic <aleksandar.markovic.sa@gmail.com>