translate: Avoid absolute value on unsigned substraction.

ast_format_get_sample_rate(.) returns an unsigned type. The difference of a
substraction between two unsigned types does not get implicitly converted to a
signed type. Therefore, using abs(.) did not make sense.

ASTERISK-27549

Change-Id: Ib904d9ee0d46b6fdd1476fbc464fbbf813304017