i2c: i2c_cdns: Fix below warnings with checker tool

This patch fixes below warnings found with checker tool.
The variable len in i2c_msg struct is of unsigned type
and it is received as recv_count which is unsigned type
but it is checked with < 0 which is always false, hence
removed it.
The local variable curr_recv_count is declared as signed
type and compared aginst unsigned recv_count which is
incorrect. This is fixed by declaring it as unsigned type.

drivers/i2c/i2c-cdns.c: In function ‘cdns_i2c_read_data’:
drivers/i2c/i2c-cdns.c:317:18: warning: comparison of
unsigned expression < 0 is always false [-Wtype-limits]
  if ((recv_count < 0))
                  ^
drivers/i2c/i2c-cdns.c:340:24: warning: comparison of
integer expressions of different signedness:
‘u32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
  updatetx = recv_count > curr_recv_count;
                        ^
drivers/i2c/i2c-cdns.c:361:39: warning: comparison of
integer expressions of different signedness:
‘u32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
    while (readl(&regs->transfer_size) !=

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>