rtc: rzn1: Fix a variable type

The calculation in ->set_offset() handles both negative and positive
offsets. The 'steps' variable will be checked to be in a specific [-x;
+x] range, which means it must be a signed integer rather than
unsigned.

This also fixes the following smatch warning:

     warn: 'steps' 'true' implies 'steps > 0' is 'true'

Fixes: be4a11cf ("rtc: rzn1: Add oscillator offset support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220520082500.489248-1-miquel.raynal@bootlin.com