clk: scpi: fix return type of __scpi_dvfs_round_rate

The frequencies above the maximum value of signed integer(i.e. 2^31 -1)
will overflow with the current code.

This patch fixes the return type of __scpi_dvfs_round_rate from 'int'
to 'unsigned long'.

Fixes: cd52c2a4 ("clk: add support for clocks provided by SCP(System Control Processor)")
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>