i2c: qup: remove redundant variables for BAM SG count

The rx_nents and tx_nents are redundant. rx_buf and tx_buf can
be used for total number of SG entries. Since rx_buf and tx_buf
give the impression that it is buffer instead of count so rename
it to tx_cnt and rx_cnt for giving it more meaningful variable
name.

Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Reviewed-by: Austin Christ <austinwc@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>